More than 1,100 people including the First Minister Peter Robinson and dignitaries from around the Commonwealth have attended a service to mark the centenary of the start of the First World War.
The First Minister joined the Prince of Wales and Prime Minister David Cameron among others for a service at Glasgow Cathedral, followed by a wreath laying ceremony at the Cenotaph.
He said he was "deeply honoured to attend today’s very poignant commemoration services".
Mr Robinson will also take part in a service in Belfast this evening, with the outbreak of the First World War remembered at a centenary commemoration to be held in St Anne's Cathedral at 7pm.
Meanwhile, Prime Minister David Cameron said the First World War had "profoundly changed our world", as he paid tribute to the sacrifice made by those who lost their lives.
Speaking outside Glasgow Cathedral ahead of a service of commemoration for Commonwealth leaders, Mr Cameron said it was important to find new ways of bringing the experiences of those involved in the conflict to life.
He told BBC Breakfast: "It was an extraordinary day in Britain's history, and when you think that almost every family, almost every community was affected... Almost a million British people were lost in this war, it's right that almost a hundred years on we commemorate it, we think about it and we mark it properly."
He added: "I think it is important to remember the outbreak of the war, because although there was an enormous amount of waste and loss of life, there was a cause that young men rallied to at the beginning of the war, which was the idea that Europe shouldn't be dominated by one power, that a country, a small country like Belgium shouldn't be simply snuffed out."
Mr Cameron said Britain entered the war because "there were important principles at stake".
He went on: "That there shouldn't be the domination of Europe by one power, that small countries had a right to their independence and their existence, and these are problems that still confront us today and some of these problems are actually the consequences of the conflicts of the 20th century - the First World War and the Second World War."
He said that, along with the terrible loss of life and suffering, the war also changed the world for better.
"The emancipation of women, the fact that women then got the vote, participated more in the workplace, there were changes in medicine, massive improvements in our world - all those things are worth remembering and that's why, as a Government and as a country, we should be refurbishing our war memorials, and we are," he said.
He added that it was also important that schools continue taking pupils to the First World War battlefields and recommended visiting the newly refurbished Imperial War Museum, which he described as "spellbinding".
Mr Cameron also said he had researched his own family's involvement in the war, and would be visiting the French battlefield where his great-great-uncle fought and died.
He described going to war as one of the "most difficult decisions that any prime minister has to take".
"Sending men and women into harm's way is a decision that you think about more than any you make as prime minister," he added.
William thanks 'steadfast' Belgium
The Duke of Cambridge thanked Belgium for its "steadfast remembrance" of those who lost their lives in the First World War, but said instability "continues to stalk our continent".
As William attended commemorations of 100 years since Britain's entry into the First World War in Belgium, he said the UK owed a great debt of gratitude to the country for its fortitude and resistance during the war.
The duke attended a ceremony in Liege, Belgium, with wife Kate to mark exactly 100 years since the country was invaded by Germany - sparking Britain's declaration of war and entry into conflict.
Over the next four years, until the armistice was signed on November 11, 1918, millions of lives were lost, including 750,000 British and Commonwealth soldiers, in what was the bloodiest conflict the world had known.
The royal couple, who are representing the Queen, were joined by dignitaries from the countries across Europe, including France, Belgium and Germany, for the ceremony at the Allies' Memorial at Cointe, overlooking the Belgian city of Liege.
Later today they will join 500 guests including Prince Harry and Prime Minister David Cameron at a twilight ceremony at St Symphorien Military Cemetery in nearby Mons.
As part of a national day of commemoration, events marking the anniversary of the start of the Great War are being held in London, Glasgow and Belgium - starting a four-year Government-led programme of remembrance.
The royal couple were welcomed to Belgium today by the country's King Philippe and Queen Mathilde ahead of the ceremony at Cointe.
Kate wore a cream coat dress with pleated skirt and Peter Pan collar and a pale hat and took her seat between French president Francois Hollande and husband William, who wore the Queen's golden and diamond jubilee medals.
Fellow dignitaries included German president Joachim Gauck, as well as Belgian Prime Minister Elio Di Rupo, who all also delivered speeches during the ceremony.
In his speech, William described how during the war Europeans were "engulfed by killing and destruction," saying: "Among the very first victims were the people of Belgium, whose resistance was as gallant as their suffering was great."
He said: "Your great sacrifice and your contribution to eventual victory was pivotal," adding: "Many nations here today, the United Kingdom among them, owe you a great debt of gratitude for your fortitude and resistance.
"Your Majesties, if I may say so, Belgium's steadfast remembrance of your war dead, and ours, is a great credit to your nation.
"On behalf of Her Majesty Queen Elizabeth, head of the Commonwealth, thank you for the honour you do us all."
He went on: "In Europe, the transition from war to lasting peace has taken time. Edith Cavell was a British nurse who saved soldiers from each side.
"On the night before she faced a German firing squad, she said: 'I must have no hatred or bitterness towards anyone'.
"It took another terrible war to learn the truth of her words, and even today we continue to learn that lesson. The events in Ukraine testify to the fact that instability continues to stalk our continent.
"The peace that we here enjoy together as allies and partners does not simply mean no more bloodshed - it means something deeper than that.
"The fact that the presidents of Germany and Austria are here today, and that other nations - then enemies - are here too, bears testimony to the power of reconciliation.
"Not only is war between us unthinkable, but former adversaries have worked together for three generations to spread and entrench democracy, prosperity and the rule of law across Europe, and to promote our shared values around the world.
"We were enemies more than once in the last century, and today we are friends and allies. We salute those who died to give us our freedom. We will remember them."
As part of the ceremony a girl, symbolising the passing on of remembrance to future generations, released a white balloon as a sign of peace and reconciliation.
At the same time thousands of other balloons in the colours of the flags of the countries invited to the commemoration were also released.
The ceremony saw wreaths laid by the King of Belgium as well as a minute's silence and The Last Post.
Music included the Belgian national anthem, the Brabanconne, performed by the Belgian Royal Air Force Band and the Concert Band of the German Armed Forces.
Earlier today, dignitaries from across the Commonwealth have been gathering in Glasgow Cathedral for a service to commemorate the centenary of the start of the First World War.
The Prince of Wales, Prime Minister David Cameron and Scotland First Minister Alex Salmond are among those attending the event, which will be led by the Reverend Laurence Whitley.
It will be followed by a wreath-laying ceremony at the city's cenotaph.
The service is the first of three events in the UK Government's programme of centenary commemorations, with a further two to be held in London and Belgium.
It will feature music, poems and readings and will have a Commonwealth theme to reflect its contribution to the war.
Those arriving at the service were handed a commemorative edition of The Flanders Fields Post.
Inside the cathedral a poppy was placed at each seat.
Lord Provost of Glasgow, Councillor Sadie Docherty, said before the service: "Glasgow is very proud to be playing a major role in the First World War centenary commemorations.
"Many politicians from across the Commonwealth, who have been in the city for the Glasgow 2014 Commonwealth Games closing ceremony, have taken up our invitation to stay on for Monday's commemorations.
"I am delighted at the prospect of our Commonwealth cousins joining me, and the rest of Glasgow, on this important day of remembrance."
Mr Salmond will later lay a wreath at the cenotaph on behalf of the people of Scotland.
Speaking before the service, he said: "No home, no school, no community in Scotland was left untouched by the devastating impact of the Great War, which remains one of the most brutal conflicts the world has ever seen.
"Between 1914 and 1918, the First World War claimed the lives of around 145,000 Scots, leaving many more thousands injured or disabled and forcing friends and loved ones across the country to come to terms with the terrible consequences.
"As the curtain falls on 2014 Commonwealth Games, we acknowledge the countries of the Commonwealth we fought alongside during the Great War and it is fitting that this service in Glasgow Cathedral should focus on the contribution of these nations."
He added: "From now until the start of 2019, we want people of all ages from all corners of Scotland, and those with Scottish connections around the world, to ask themselves and each other what can be learned from the Great War.
"Our journey of commemoration starts today in Glasgow with the Commonwealth-themed service at Glasgow Cathedral and cenotaph as part of the UK Commemorations Programme, and it will continue on August 10 when we start the Scottish Commemorations Programme with a drumhead service, procession and memorial in Edinburgh."
Mr Salmond also highlighted Legion Scotland's Voice of Veterans campaign to help preserve stories of military life.
"I would encourage everyone who remembers hearing stories from those who experienced life during the Great War to contact Legion Scotland to be a part of this important project," he said.
Switch off the lights tonight, and remember the darkness that fell in 1914, writes Maureen Coleman
Homes and businesses across Northern Ireland are being urged to switch off lights and lamps for one hour tonight to commemorate the 100th anniversary of the outbreak of the First World War.
In a symbolic, nationwide gesture, houses and buildings throughout the UK will be plunged into darkness at 10pm, with just a single light or candle burning for an hour.
The dimming of the lights is just one of a number of events taking place to mark 100 years since Prime Minister Herbert Asquith declared that Britain was at war with Germany on August 4, 1914.
Tens of thousands of house-holders and hundreds of buildings, including the Houses of Parliament and St Paul's Cathedral in London, are expected to join in the Lights Out plan.
Closer to home, special events are being held in Northern Ireland to mark the start of the conflict, which claimed the lives of millions of soldiers from around the world.
In Belfast, the outbreak of the First World War will be remembered at a centenary commemoration service to be held in St Anne's Cathedral, tonight at 7pm.
The service is expected to be attended by Secretary of State Theresa Villiers, Heather Humphreys TD, Minister for Arts, Heritage and the Gaeltacht for the Republic, and senior figures from the main churches in Northern Ireland.
The Queen will be represented in Belfast by the Duke of York, who will read a lesson and will light a candle to mark the occasion.
The service will be conducted by the Dean of Belfast, the Very Rev John Mann. Young people will light five candles during the service, each representing a year of the war.
Admission to the event is by invitation only.
Elsewhere in Northern Ireland, a number of candlelit vigils are being held to mark the occasion. In Coleraine, a vigil will be held in the town centre at 10.30pm, followed by a tolling of church bells at 11pm, while in Carrickfergus, the Knockagh Monument will be lit up for an hour at 10pm.
In Antrim, a candlelit vigil will be held followed by the release of lanterns into the sky.
Meanwhile, the great-niece of a First World War soldier will join dignitaries, including the Duke and Duchess of Cambridge, at a special ceremony this evening.
Helen Jones will be the first member of her family to visit the grave of her great-uncle Private George Bellamy at St Symphorien military cemetery near Mons in Belgium.
Mrs Jones and her husband will be among a guest list which includes the Duke and Duchess of Cambridge and Prince Harry.
The 57-year-old plans to lay some flowers on the grave of Private Bellamy, of 2nd Battalion the Royal Irish Regiment, to say thank you for the sacrifice made by him and millions of other soldiers.
The mother-of-three, who will be joined on the trip by her husband, said she expected it to be an emotional occasion.
"I really want to lay some flowers on George's grave and just say 'you know what, thanks, because you changed the world for us'. Without it, it wouldn't be the same," she said.
"They just gave everything, a whole generation almost of young men just disappeared. We just can't credit it."