The Prime Minister has paid tribute to Lyra McKee, the 29-year-old journalist murdered on the streets of Derry during rioting.
Theresa May said: "The death of Lyra McKee in last night's suspected terrorist incident in Londonderry is shocking and truly senseless.
"My deepest condolences go to her family, friends and colleagues. She was a journalist who died doing her job with great courage."
US speaker of the House of Representatives Nancy Pelosi, speaking during a visit to Northern Ireland, and Irish Taoiseach Leo Vardakar, both extended their condolences to Lyra's family and friends.
Police are investigating the murder as a terrorist incident.
Lyra had been standing close to a police Land Rover when a gunman opened up during disorder on Thursday night.
Police said they believed dissident republicans were behind the night of disorder in the Creggan area blaming the new IRA.
In an early morning press conference Assistant Chief Constable Mark Hamilton said police had information of planned attacks in the city over the Easter weekend and had carried out searches for munitions and firearms.
Mr Hamilton offered his sympathies to the family of Ms McKee and appealed for anyone with information to contact his officers.
He said her death was "horrendous and unjustified".
Politicians have widely condemned the killing.
Sinn Fein deputy leader Michelle O’Neill said the killing in Derry was a “senseless loss of life”.
She said: “I am shocked and saddened at the tragic news that a young woman has been shot dead by so-called dissidents in the Creggan estate tonight.
“The murder of this young woman is a human tragedy for her family, but it is also an attack on all the people of this community, an attack on our peace process and an attack on the Good Friday Agreement.
“I unreservedly condemn those responsible for killing this young woman.
“We will remain resolute in our opposition to the pointless actions of these people who care nothing for the people of Derry.”
Democratic Unionist Party leader Arlene Foster tweeted: “Heartbreaking news. A senseless act. A family has been torn apart.
“Those who brought guns onto our streets in the 70s, 80s & 90s were wrong. It is equally wrong in 2019.
“No one wants to go back. My thoughts are also with the brave officers who stood in defence of their community.”
A grotesque crime. Colum Eastwood
SDLP leader Colum Eastwood MLA said the murder was a "grotesque crime".
He said: "On behalf of the SDLP, I want to extend our thoughts and prayers to Lyra McKee’s family this morning. They are experiencing the unthinkable loss of their daughter in the most heartbreaking circumstances.
“We have all lost an incredible young talent. I had the opportunity to speak with Lyra on a number of occasions as a journalist, as will politicians and leaders from all parties and communities. She had a passionate thirst for the truth and for telling the stories of people who had been forgotten or abandoned."
He continued: "Her character and values stand in stark contrast to the cowards who fired indiscriminately at police officers in Derry last night and have murdered a young woman who had so much to contribute to shaping our society and bringing people together.
"This news, coming on Good Friday, is a dark reminder to us all that our peace is fragile and that we must protect it everyday from those who want to shatter the progress that we have made. Those responsible for this heinous crime need to be faced down, they need to be made to realise that their fight is not with the PSNI or with young journalists doing their job. Their fight is with all of us, the people of Ireland. And it is a fight that they will never win.
“Today our thoughts are with Lyra’s family and friends. We should all remember her passion, humour and thirst for justice."
Taoiseach Leo Vardakar tweeted: "The Government condemns in the strongest possible terms the fatal shooting of journalist and writer Lyra McKee in Derry.
"We are all full of sadness after last night's events. We cannot allow those who want to propagate violence, fear and hate to drag us back to the past.
"I would like to express my sincere condolences to Lyra's family, her partner Sara and her friends.
"Our solidarity also goes out to the people of Derry and to the entire journalism community."
US politician Nancy Pelosi, during a speech at Stormont, said: "Sadly, we join you, on the sadness, the tragedy that happened last night, we extend our condolences to the family, coming on Good Friday, it is especially poignant."
Gary Middleton, a DUP former deputy mayor of Derry City Council and current Member of the Legislative Assembly, tweeted: “Disgraceful scenes of violence in the Creggan estate tonight.
“PSNI and members of the public being put in danger by unrepresentative thugs intent on destroying communities.”
UUP leader Robin Swann tweeted: "Devastating news coming out of Londonderry (...), a precious life lost, we can't go back to this nor can we allow others to drag us back."
Mark H Durkan, SDLP MLA for Foyle, said he was left “heartbroken and angry” by the killing.
“Just leaving Creggan, heartbroken and angry at the senseless loss of a young life,” he tweeted.
“Violence (only creates victims, that’s all it ever has done. The thoughts and prayers of our city are with the young woman’s family and friends, may she rest in peace.”
The Secretary of State for Northern Ireland, Karen Bradley added: "I am deeply shocked and saddened to hear of the death of a woman in Londonderry last night.
"My thoughts and condolences are with her family at this time. Those responsible for last night's violence have nothing to offer anyone in Northern Ireland.
"Their intolerable actions are rejected by the overwhelming majority of people who want to build a peaceful and more prosperous future for everyone in Northern Ireland."