Labour MP Jo Cox has died after she was shot and stabbed on a street in her West Yorkshire constituency.
The 41-year-old, who was married with two young daughters aged three and five, was left lying bleeding on the pavement in Birstall, near Leeds.
One witness said she was also kicked during the attack, while another reported three shots, including one "round the head area".
A 52-year-old man has been arrested. He has been named locally as Tommy Mair, and police say they are not looking for anyone else.
The attack is believed to have taken place as Mrs Cox came out of the town's library, where she had been holding an advice surgery and meeting constituents.
Her husband Brendan released a moving statement: "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 poisionous."
Mr Cox had earlier tweeted a picture of the former senior Oxfam activist near Tower Bridge, close to where the family have a houseboat.
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Mrs Cox, who was elected as MP for the Batley and Spen seat in last year's general election, was taken to hospital in a critical condition following the attack but police later confirmed she had died.
A 77-year-old man suffered non-threatening injuries in a nearby incident, according to officers.
Eyewitness Hichem Ben Abdallah said a "very brave" bystander tried to stop a man who shot Mrs Cox twice and assaulted her.
Some reports said that two men had been involved in an argument in the street before Ms Cox intervened and was attacked.
Mr Abdallah, who was in a cafe next door to the library, told Sky News he heard two shots.
He saw a man wearing a "dirty white baseball cap" who started "jostling with somebody", a bystander who appeared to be trying to stop him.
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He 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."
Mr Abdallah said Mrs Cox was shot from between two cars and then kicked as she lay on the ground.
"It looked like a gun from, I don't know, the First World War or a makeshift, handmade gun. It's not sort of like the kind of gun you see normally."
:: MP's Killer Walked Off Coolly - Witness
Mr Abdallah described a hysterical situation with lots of people screaming, but added that the gunman walked off "very, very coolly, very slowly".
Constituent Susan Baxter paid tribute to Mrs Cox, telling Sky News: "She was for the people, you could go to her with anything. She was like a big sister or a surrogate mum. This is heartbreaking."
Hairdresser Claire Saville said: "She absolutely understood you, you wouldn't think she was an MP. Why do that to a defenceless woman?
"She's got children, that's all I can think of at this time. She's gone to work to do a job she cares about and now she's not coming home to them. I'm devastated."
A vigil has been held at St Peter's Church in Birstall, and flowers have been laid alongside a statue of 18th century theologian Joseph Priestley in the town.
One tribute read: "To our dear Jo, snatched so cruelly from us but loved and remembered always."
Mrs Cox is the first MP to be murdered in office since Conservative politician Ian Gow was killed by an IRA car bomb in 1990.
Labour leader Jeremy Corbyn said she was a "much-loved colleague" who "died doing her public duty at the heart of our democracy, listening to and representing the people she was elected to serve".
He later led a vigil surrounded by colleagues in Parliament Square, telling reporters that the politician's death was "beyond appalling".
Prime Minister David Cameron said: "We've lost a great star. She had a big heart and people are going to be very, very sad at what has happened."
The EU referendum Remain and Leave campaigns suspended their activities for Thursday and Friday as a result of the attack, and Mr Cameron cancelled a pro-EU rally in Gibraltar.
In the hours after the attack, The Guardian reported that a man was being interviewed by police over an abusive phone call made to the office of MP Ben Bradshaw.