Vigils have been held to remember Labour MP Jo Cox who died after being shot and stabbed in the street outside her constituency advice surgery.
The mother of two children aged three and five was attacked by a man reportedly shouting "Britain first" at lunchtime on Thursday in Birstall, West Yorkshire.
Eyewitnesses said he kicked and stabbed her and then shot her several times, the final shot aimed at her head.
The alleged gunman has been named locally as Tommy Mair, 52, who neighbours in Birstall have described as "a loner". He was arrested near the scene soon after the attack.
The MP's husband Brendan has vowed to work against the hate that killed his wife
He said: "Today is the beginning of a new chapter in our lives. More difficult, more painful, less joyful, less full of love.
"I and Jo's friends and family are going to work every moment of our lives to love and nurture our kids and to fight against the hate that killed Jo.
"Jo believed in a better world and she fought for it every day of her life with an energy and a zest for life that would exhaust most people.
"She would have wanted two things above all else to happen now: one, that our precious children are bathed in love and two, that we all unite to fight against the hatred that killed her.
"Hate doesn't have a creed, race or religion, it is poisonous.
"Jo would have no regrets about her life, she lived every day of it to the full."
Labour leader Jeremy Corbyn said the country would be "in shock at the horrific murder" of the MP, who was a "much-loved colleague".
The killing shocked Westminster and led to the suspension of campaigning in the EU referendum.
Mr Corbyn and a number of other MPs attended an impromptu vigil in Parliament Square and flowers were laid nearby in tribute.
In Mrs Cox's home town, hundreds of people, including Labour colleagues Yvette Cooper and Dan Jarvis, packed into the parish church to hear the Bishop of Huddersfield, the Rt Rev Dr Jonathan Gibbs pay tribute to someone who "gave her life for this community".
Prime Minister David Cameron said: "The death of Jo Cox is a tragedy. She was a committed and caring MP. My thoughts are with her husband Brendan and her two young children."
Downing Street said flags across Whitehall will fly at half mast while flags at Buckingham Palace and the Palace of Holyroodhouse in Edinburgh will also be lowered in tribute.
A Buckingham Palace spokesman said the Queen had written privately to Mr Cox.
Witness Clarke Rothwell, who runs a cafe near the murder scene, described the attack, telling the Press Association: "He was shouting 'put Britain first'. He shouted it about two or three times. He said it before he shot her and after he shot her."
He said the gunman fired three shots, the final one at her head.
Britain First is the name of a far-right group which said it was "not involved and would never encourage behaviour of this sort".
Another witness, Hichem Ben Abdallah, 56, said the attacker was "kicking her as she was lying on the floor".
He said that after a bystander intervened, the man produced a gun, stepped back and shot Mrs Cox.
Mr Abdallah, who was in the cafe next door to the library, told the Press Association: "There was a guy who was being very brave and another guy with a white baseball cap who he was trying to control and the man in the baseball cap suddenly pulled a gun from his bag.
"He was fighting with her and wrestling with her and then the gun went off twice."
He added: "I came and saw her bleeding on the floor."
Mr Abdallah said the weapon looked handmade and that the man who had been wrestling with the assailant continued to do so even after he saw the gun.
He said: "The man stepped back with the gun and fired it and then he fired a second shot. As he was firing he was looking down at the ground.
"He was kicking her and he was pulling her by her hair."
Mair's house was sealed off by police who were guarding the property as forensic officers worked in the garden.
Neighbours said he had lived there for more than 30 years - on his own for the last two decades since the death of his grandmother.
One neighbour, David Pickles, said: "He kept himself to himself."
Another neighbour, who did not want to be named, said: "He was a bit of a loner. It's a bit of a shock - he did everyone's gardens."
Leah Ainley, who lives in Risedale Avenue, where Mair was arrested, said armed police surrounded him and pinned him to the ground.
"I just saw them take him down and catch him," she added.
"The police were armed. There were more than 10 of them at the end."
Mrs Cox, a Remain supporter, was elected to the seat of Batley and Spen at the general election last year.
Shortly after the attack Mr Cox tweeted a picture of his wife next to the River Thames, where they lived on a houseboat.
She graduated from Cambridge University in 1995 and worked as an adviser for former Labour prime minister Gordon Brown's wife Sarah, and Baroness Kinnock.
Mrs Cox was a vocal advocate for the victims of the Syrian civil war and abstained in last autumn's contentious vote on allowing British military action in Syria.
Mr Cox, a former adviser to ex-prime minister Gordon Brown, spent Wednesday campaigning for the UK to remain in the EU.
He posted photographs of himself and the couple's children travelling along the Thames in a dinghy during a counter-protest against a pro-Brexit flotilla of vessels.