“Today is the beginning of a new chapter in our lives,” Brendan Cox said in a statement overnight.

“More difficult, more painful, less joyful, less full of love.

The Labour MP, an advocate of the Remain campaign ahead of next week’s Brexit vote, was attacked Thursday afternoon (late last night, Australian time) in Birstall, Acting Chief Constable Dee Collins of West Yorkshire Police said.

She was pronounced dead by a doctor less than an hour later.

A 77-year-old man was injured.

Tommy Mair, 52, has been arrested and Collins said police believe it is a “lone incident” and are not looking for anyone else in connection with the attack.

Media reports citing witnesses said the attacker had shouted out “Britain First”, which is the name of a right-wing group that describes itself on its website as “a patriotic political party and street defence organisation”.

Police at the scene in Birstall, West Yorkshire, after Batley and Spen MP Jo Cox was shot. Photo: Nigel Roddis, PA Wire.

Jayda Fransen, deputy leader of Britain First, said the attack was “absolutely disgusting” and suggested Britain first was a common slogan being used in the EU referendum campaign by those who support Brexit.

“We were as shocked to hear these reports as everyone else,” Fransen said.

“At the moment would point out this is hearsay, we are keen to verify the comments but we can only do that when the police provide more details.”

Cox’s husband Brendan said: “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,” he said.

“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 whole of the Labour Party and Labour family – and indeed the whole country – will be in shock at the horrific murder of Jo Cox today.”

Both the Vote Leave and Britain Stronger in Europe campaigns suspended activity ahead of next week’s vote over whether Britain should remain a part of the 28-member bloc at the news.

Violence against politicians is rare in Britain, as are gun attacks, and figures from all parts of the political spectrum expressed deep shock.

Prime Minister David Cameron said he was cancelling a planned speech in Gibraltar after “the terrible attack.”

“Our thoughts and prayers are with Jo and her family,” Cameron tweeted.

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.

— David Cameron (@David_Cameron) June 16, 2016

Britain’s Press Association news agency quoted eyewitness Hithem Ben Abdallah as saying Cox got involved in a scuffle between two men in Birstall, 320 kilometers north of London.

Abdallah said one of the men was fighting with Cox and then a gun went off twice and “she fell between two cars and I came and saw her bleeding on the floor”.

Clarke Rothwell, who runs a nearby cafe, told the BBC he believed Cox had been shot and stabbed multiple times.

“Three times she was shot, the initial time which then she dropped to the floor and two more times,” he said.

“The third time he got close proximity he shot her around the head area.”

“In the meantime he was stabbing her as well. He was stabbing her with his knife.”

Items on the ground at the crime scene. Photo: Nigel Roddis, PA Wire.

Shopkeeper Sanjeev Kumar told the BBC said he saw a woman lying on the ground “bleeding from the mouth and nose”, with two women trying to help her.

After around 15 minutes, witnesses said emergency services arrived and tended to her with a drip.

Cox, a former worker for charities who has two young children, was elected to the House of Commons in the May 2015 general election.

She has been one of the most outspoken MPs on the Syrian civil war and has been critical of Britain’s reluctance to deepen its military involvement against the Islamic State group as part of efforts to end it.

PA reported Home Secretary Theresa May as calling the killing a “dreadful event” and adding that Westminster had lost one of its “brightest and most popular” MPs.

Former Labour MP Joan Walley, in whose parliamentary office Ms Cox had worked before becoming an MP herself, was being interviewed on BBC Radio as the news came in.

A clearly distressed Ms Walley said: “Nothing has prepared me to be in a situation where I’m live on BBC radio to pay tribute to a really, truly special and compassionate woman, who totally believed in public service, who totally believed in eradicating poverty, who totally wanted to see environmental improvements, with a young family and a whole life in politics in front of her.”

Neighbours of the man held in relation to the fatal attack described him as a loner and quiet.

Neighbour David Pickles described Mair as the last person he would have thought of being involved, saying he was quiet, kept to himself and had lived on his own for about 20 years.

As Australians awoke to the news, Liberal frontbencher Christopher Pyne said MPs should not hide in their shells or stop meeting constituents.

Pyne says one of the great aspects of democracy in Australia and in Britain is the connection politicians have with the public, describing the murder as a terrible tragedy.

“It is important … that we don’t change the way we behave,” he told the Nine Network.

Pyne said the murder highlighted the vulnerable position MPs often found themselves in without security.

“It just really underlines how careful we all have to be.”

Prime Minister Malcolm Turnbull said he was deeply shocked by Ms Cox’s death.

“Our condolences, prayers and solidarity are with her family & the people of the UK,” he tweeted.

Deeply shocked by the murder of UK MP Jo Cox. Our condolences, prayers and solidarity are with her family & the people of the UK.

— Malcolm Turnbull (@TurnbullMalcolm) June 16, 2016

Labor frontbencher Anthony Albanese described it as a hate crime.

-PA, AP, AAP, Reuters