Sinn Fein leader Michelle O'Neill has paid tribute to Martin McGuinness following his death, describing him as a "giant of Irish politics".
Mr McGuinness, 66, died during the night at Derry's Altnagelvin Hospital.
Over the course of his political career he transitioned from IRA commander to being the face of Sinn Fein in Northern Ireland and an influential figure in the peace process.
Michelle O'Neill, who succeeded Mr McGuinness as his party's leader at Stormont, said: "Martin was truly a giant of Irish politics and was known and respected across the world.
"He led republicanism from the front for decades, striving for reunification and promoting peace and reconciliation and a genuine commitment to equality and respect."
DUP leader Arlene Foster said: “History will record differing views and opinions on the role Martin McGuinness played throughout the recent and not so recent past, but history will also show that his contribution to the political and peace process was significant.”
Prime Minister Theresa May said he played a defining role in moving republicans away from violence.
The former IRA commander was a key architect in delivering the Good Friday Agreement in 1998 and in recent years made further history as he met the Queen on a number of occasions.
He served as deputy First Minister with three DUP First Ministers - the late Ian Paisley, Peter Robinson and Mrs Foster, before he left active politics in January.
Ian Paisley, son of the late Lord Bannside who developed a close bond with Mr McGuinness, paid his respects saying: "It's not how you start your life, it's how you finish your life".
The North Antrim MP said he had gone from viewing Mr McGuinness as the "godfather of the IRA" to considering him a personal friend.
"And most people will be thankful that Martin McGuinness finished his life a lot better than it could have been."
Michelle O'Neill: 'A giant of Irish politics'
My heart is broke this morning. We have lost a legend, a giant of a man. I'm very proud to say he was my friend and mentor x
Mrs O'Neill said: "On behalf of Sinn Fein I want to offer my deepest condolences following the death of our friend and comrade, Martin McGuinness.
"Martin was truly a giant of Irish politics and was known and respected across the world.
"He led republicanism from the front for decades, striving for reunification and promoting peace and reconciliation and a genuine commitment to equality and respect.
"Growing up as a young republican I was inspired by Martin McGuinness and I continue to be today.
"His leadership, grace and warm personality have transformed Irish politics for the better and his impact will be felt for many years to come.
"My thoughts are with his wife, Bernie, and children, Emmet, Fiachra, Fionnuala and Grainne and the entire clann at this time."
Arlene Foster: 'Pivotal'
Ms Foster said: “He served the people of Northern Ireland as deputy first minister for nearly a decade and was pivotal in bringing the republican movement towards a position of using peaceful and democratic means.”
“In recent years his contribution helped build the relative peace we now enjoy,” Ms Foster said.
“While our differing backgrounds and life experiences inevitably meant there was much to separate us, we shared a deep desire to see the devolved institutions working to achieve positive results for everyone. I know that he believed that the institutions were the basis for building stability.
Arlene Foster said she wanted to acknowledge the contribution by Martin McGuinness to the governance of Northern Ireland pic.twitter.com/QA1MOZ8uAJ
She described Mr McGuinness as a “much loved husband, father and grandfather”.
“Today's news will come as a shock to many people.
“My thoughts and prayers are with his wife and the family circle at this very painful time of grief and loss,” she added.
“Martin faced his illness with courage and, after stepping away from the glare of the public spotlight I sincerely hope he got the chance to enjoy the things he loved."
Bill Clinton: 'His word was gold'
Former American president, Bill Clinton said he and his wife Hillary were "saddened" to learn of the death.
"When he decided to fight for peace, Martin was calm, courageous, and direct. And when he gave his word, that was as good as gold.
"As Sinn Fein’s chief negotiator, his integrity and willingness to engage in principled compromise were invaluable in reaching the Good Friday Agreement. In the years that followed, he played an even more important role in ensuring that the peace would last—personally overseeing the arms decommissioning, joining the new government as the first Education Minister, and later serving as Deputy First Minister, and doing it all with a sense of humor and fairness that inspired both his friends and former foes.
"My lasting memory of him will be the pride he took in his efforts to improve disadvantaged schools in Unionist and Protestant communities. He believed in a shared future, and refused to live in the past, a lesson all of us who remain should learn and live by. May he rest in peace."
John and Pat Hume: 'An indelible print on politics in Ireland'
In a statement, former SDLP leader John Hume and his wife Pat said: “Martin McGuinness left an indelible print on politics in Ireland. Our starting points on the approach to finding a solution to the divisions on this island were very different but there can be no doubt Martin was deeply committed to the agreed Ireland of the Good Friday Agreement and its power sharing institutions.
"He played a very important role in our peace process and his courageous, generous and gracious gestures as deputy First Minister were offered in a spirit of reconciliation and peace building.
“Our deepest condolences go to his wife Bernie, their children Fiachra, Emmet, Grainne, Fionnuala and the wider family circle.”
Theresa May: 'Defining role in leading republicans from violence'
Mrs May said: "First and foremost, my thoughts are with the family of Martin McGuinness at this sad time.
"While I can never condone the path he took in the earlier part of his life, Martin McGuinness ultimately played a defining role in leading the republican movement away from violence. In doing so, he made an essential and historic contribution to the extraordinary journey of Northern Ireland from conflict to peace.
"While we certainly didn’t always see eye-to-eye even in later years, as deputy First Minister for nearly a decade he was one of the pioneers of implementing cross community power sharing in Northern Ireland. He understood both its fragility and its precious significance and played a vital part in helping to find a way through many difficult moments.
"At the heart of it all was his profound optimism for the future of Northern Ireland – and I believe we should all hold fast to that optimism today."
Despite our differences I enjoyed working with Martin McGuinness. He wanted politics to work in Northern Ireland & helped make it happen.
Earlier Sinn Fein President Gerry Adams said Mr McGuinness “showed great determination, dignity and humility and it was no different during his short illness.”
“He was a passionate republican who worked tirelessly for peace and reconciliation and for the re-unification of his country. But above all he loved his family and the people of Derry and he was immensely proud of both.”
James Brokenshire: ‘Passionate and robust in his politics’
Secretary of State for Northern Ireland James Brokenshire said Mr McGuinness will be remembered for his contribution to politics in Northern Ireland.
“No-one can doubt the essential role he played in helping to secure the power sharing arrangements and political progress in Northern Ireland,” Mr Brokenshire said.
“Martin’s personal journey and the clear influence he had on others in the Republican movement were instrumental in shaping political institutions in Northern Ireland founded on exclusively peaceful and democratic means.
“While not forgetting the past, Martin's commitment to reconciliation and understanding across communities was a significant factor.”
He added: “Whilst passionate and robust in his politics, on a personal level I always found Martin to be thoughtful and reflective and appreciated the personal consideration he showed. The importance of family and his home in Derry shone through.”
Michael D. Higgins: 'Unfailing courtesy'
Irish President Michael D. Higgins said: “The world of politics and the people across this island will miss the leadership he gave, shown most clearly during the difficult times of the peace process, and his commitment to the values of genuine democracy that he demonstrated in the development of the institutions in Northern Ireland.
“As President of Ireland, I wish to pay tribute to his immense contribution to the advancement of peace and reconciliation in Northern Ireland – a contribution which has rightly been recognised across all shades of opinion.”
Martin McGuinness played a huge role in bringing about peace in Northern Ireland. He was a great family man and my thoughts are with them
“His death leaves a gap that will be difficult to fill."
The two men were candidates in the Irish presidential election campaign in 2011.
“As a political colleague of many years, and having participated together in the Presidential election campaign of 2011 that brought us all over Ireland, Sabina and I have appreciated both Martin McGuinness’ warmth and his unfailing courtesy."
Enda Kenny: 'Commitment to peace unwavering'
Taoiseach Enda Kenny said: "His passing represents a significant loss, not only to politics in Northern Ireland but to the wider political landscape on this island and beyond.
"Martin will always be remembered for the remarkable political journey that he undertook in his lifetime. Not only did Martin come to believe that peace must prevail, he committed himself to working tirelessly to that end.
"Martin was one of the chief architects of the Good Friday Agreement and he worked resolutely in the years that followed it in pursuit of its full implementation.
"I got to know Martin well in recent years, including through our working together in the North South Ministerial Council. His commitment to securing enduring peace and prosperity for all of the people of Northern Ireland was unwavering throughout this time.
"He strove to make Northern Ireland a better place for everyone, regardless of background or tradition.
"Above all, today is an especially sad day for Martin's family - his wife Bernie, and his children - and for the people of Derry, who held a special place in Martin's heart.
"My deepest sympathies are with all of them at this sad and difficult time."
Sad to learn of sudden passing of Martin McGuinness. His contribution to peace in Northern Ireland will endure. Ar dheis Dé go raibh a anam.
Tony Blair: ‘McGuinness set aside that armed struggle in favour of making peace’
Former Prime Minister Tony Blair said he first met Mr McGuinness in Northern Ireland and “then again shortly after in Downing Street - an historic meeting, between a British PM and the Republican leadership in the Cabinet room where so much Irish history had been made”.
Mr Blair said: “I grew up watching and hearing about the Martin McGuinness who was a leading member of the IRA engaged in armed struggle. I came to know the Martin McGuinness who set aside that armed struggle in favour of making peace.
“There will be some who cannot forget the bitter legacy of the war. And for those who lost loved ones in it that is completely understandable. But for those of us able finally to bring about the Northern Ireland peace agreement, we know we could never have done it without Martin's leadership, courage and quiet insistence that the past should not define the future.”
“Over the years - through the arduous negotiation of the Good Friday Agreement - and for the years after it, I got to know Martin well. We met many, many times and as the trust grew between himself, my team, Gerry Adams and their team, so the discussions became increasingly open, frank and therefore productive.
“By the time that extraordinary day arrived in 2007 after almost a decade of hard work where we could witness the - to my generation - incredible sight of he and Ian Paisley sitting down together in Government, the transition of Martin to reconciliator was complete.”
SDLP leader Colum Eastwood also reflected “on Martin’s remarkable journey, made possible by men and women from all traditions across this island who forged a peace process from the fire of a terrible conflict.”
“The loss of Martin McGuinness is a significant moment in the history of this island but it is, first and foremost, a devastating loss to his family, friends and colleagues. Our thoughts are with them now,” he said.
“History will record his political career as a journey – one born in a tradition of violence but, in a testament to Martin’s character, that arrived at his true calling in politics, people and the art of persuasion.
“Those who knew him will know that his warm and affable nature undoubtedly made it easier to reach beyond his own political base. The generosity that he displayed in developing relationships with Ian Paisley and Peter Robinson demonstrated a rare gift that came as much from his personality as his politics. It is that gift which is needed in our politics at this moment.”
Mike Nesbitt: 'Challenging day for victims’
Ulster Unionist leader Mike Nesbitt said this will be “a very challenging day for victims of the Troubles”.
“I believe no-one needed to die to get Northern Ireland to where it is today. Clearly Martin McGuinness very actively disagreed with that analysis, but I also accept in his later years he was on a journey to create change through politics, becoming a pivotal figure at Stormont.
Mr Nesbitt said he found Mr McGuinness “a straight-dealing politician in any engagement I had with him”.
“We must recognise the loss to the McGuinness family and I extend my sympathies to them. Like any family they need time and space to mourn.
“History will reflect a complex life story.”
Naomi Long: 'Grateful’
Alliance leader Naomi Long paid tribute to “the hard work and dedication Martin invested as an MLA and as deputy First Minister to serving not only his constituents, but all of Northern Ireland.”
“Whilst our politics were very different and his past is well documented, the compromises he made, the leadership he demonstrated and his willingness to work with others despite those differences as part of the peace process helped secure the peace we all now enjoy,” she said.
“For that, we are grateful and our best tribute to him would be to do all in our power to secure that peace and progress for future generations.”
Chief Constable George Hamilton: 'Martin McGuinness believed in a better future'
Mr Hamilton said: "I was saddened to hear of the death of former Deputy First Minister, Martin McGuinness. I extend the sympathies of the Police Service to his wife and family at this sad time.
"Our society suffered grievously throughout our troubled history; and many police officers and their families are among those who suffered.
"But Martin McGuinness believed in a better future for our community; and this is a vision shared by policing.
"Martin’s journey in life challenges all of us who care about the future; to be prepared to change; to demonstrate leadership; and to work to understand the world, not just from our own perspective, but from the perspectives of everyone in our community."
Civil Service head Sir Malcolm McKibbin: 'Loss acutely felt by Stormont staff'
"It is with sadness that I have learnt of the death of former deputy first minister Martin McGuinness," he said.
"Although his recent health issues were well documented this news will still come as a shock to many people.
"Martin was always very supportive and courteous to me as head of the Northern Ireland Civil Service and he was tireless in his efforts to improve the workings of the devolved institution.
"There will be many tributes paid by political leaders both at home and abroad today, but Martin's loss will also be acutely felt by his support staff who worked closely with him here in Stormont Castle.
"Martin was hugely appreciative of all their efforts and regardless of what was in the diary for the day, Martin always made time for them.
"I extend my condolences to his wife Bernie, his children, grandchildren and the wider McGuinness family."
1/2 Very sorry to hear about the passing of Martin McGuinness.Look back with pleasure on the remarkable year he and my father...