UPDATED 7.02pm: Vigils have been held to remember British Labour MP Jo Cox who died after being shot and stabbed in the street outside her constituency advice surgery.
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The 41-year-old mother of two daughters aged three and five was attacked by a man reportedly shouting "Britain first" at lunchtime on Thursday in Birstall, West Yorkshire.
Eyewitnesses say 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 Cox 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.
Corbyn and a number of other MPs attended an impromptu vigil in Parliament Square and flowers were laid nearby in tribute.
In Jo Cox's home town, hundreds of people, including her Labour colleagues, packed into the parish church to hear the Bishop of Huddersfield pay tribute to someone who "gave her life for this community".
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 Brendan Cox.
Witness Clarke Rothwell, who runs a cafe near the murder scene, described the attack: "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 Jo Cox.
Abdallah, who was in the cafe next door to the library, said: "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."
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.
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.
Jo Cox, a Remain supporter, was elected to the seat of Batley and Spen at the general election last year.