UPDATED 8.25am: The man arrested over the death of British Labour MP Jo Cox was a loner with alleged links to hard-right groups, according to British media reports.

Ms Cox, 41, died after being repeatedly shot and stabbed by a man in her constituency in northern England.

The opposition MP, a mother of two children, was attacked at about 1pm Thursday local time (10pm AEST) as she prepared to hold a meeting with voters in a library on Market Street, Birstall, West Yorkshire.

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The man held over the fatal attack has been described by neighbours as a “loner”.

Tommy Mair, 52, was detained by police near his home in Birstall, West Yorkshire, while officers were seen searching his house in Birstall not far from the crime scene.

Britain’s Independent newspaper has reported that Mair was believed to have had long-term links with a hard-right group based in London which had been campaigning for many years for Britain to leave the European Union.

The paper has linked Mair to an online publication of the Springbok Club, an organisation which has defended the white supremacist apartheid regime in South Africa.

Witnesses to the fatal attack on Ms Cox say they heard Mair shout “Britain First” as the MP was shot and then stabbed.

Thomas Mair, 52, has been arrested. Photo: SWNS.

The organisation Britain First, which was founded by former members of the British National Party, has denied that Mair was associated with it and say they condemn the killing.

Neighbours said Mr Mair had lived there for more than 30 years – on his own for the last two decades since the deaths of his mother and grandmother.

In 2010 a Thomas Mair, then aged 46, was quoted in his local paper talking about his mental health issues.

He volunteered at the Oakwell Hall country park in Birstall in 2010 after being a patient of the Mirfield-based Pathways Day Centre for adults with mental illness, according to a Huddersfield Examiner report at the time.

He told the paper: “I can honestly say it has done me more good than all the psychotherapy and medication in the world.

“Many people who suffer from mental illness are socially isolated and disconnected from society, feelings of worthlessness are also common, mainly caused by long-term unemployment.”

Neighbour David Pickles said: “He’s lived there longer than me and I’ve lived here since 1975. I still can’t believe it. He’s the last guy I would have thought of.

“He’s just quiet. He kept himself to himself. He lived by himself. He’s been on his own for about 20 years.

“I’ve never seen a lot of people visiting or anything like that but he likes gardening. He did a lot of people’s gardens round here. But he did it quietly.”

Mr Pickles said he did not know of any connection between his neighbour and Ms Cox.

Another neighbour, who did not want to be named, said: “We’ve known him quite a lot of years. I’ve known him since I was young. He lived with his grandmother in that house, she died a few years ago. He was a bit of a loner. It’s a bit of a shock – he did everyone’s gardens.

“I can’t say a wrong word about him, he was so quiet. It’s come as a shock to everybody.”

Leah Ainley, who lives in Risedale Avenue where Mr 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. I just looked out the window and saw them. The man was white and bald. He banged his head on the floor. He just lay on the floor while they were holding him down.

“He had a bag with him but that’s all I saw, I didn’t see if he was armed.”

Ms Cox succumbed to her “serious and sadly, ultimately fatal injuries” at the scene, a police spokeswoman said.

“I am now very sad to have to report she has died as a result of her injuries,” West Yorkshire chief constable Dee Collins said during a press conference.

“She was pronounced dead by a paramedic crew at the scene.”

“It is too early to comment on any motive. We are not looking for anyone else in connection with the incident.”

Ms Cox was found bleeding on this footpath in West Yorkshire. Photo: AAP

An eyewitness, Hichem Ben Abdallah, 56, told local media that he saw a man in a white baseball cap kick Ms Cox and shoot her with what appeared to be a handmade firearm.

Other eyewitnesses described it as an old-style handgun or musket, and said he also wielded a large knife.

“He was fighting with her and wrestling with her and then the gun went off twice and then she fell between two cars and I came and saw her bleeding on the floor,” Mr Abdallah told the Press Association.

The gunman shouted “Britain First” many times during the attack, The Guardian reported, citing multiple eyewitnesses. The phrase may be a reference to a far-right political party.

As a result of Ms Cox’s death, the Vote Leave and Remain movements have both suspended campaigning in the EU referendum in light of the attack.

Mrs Cox, 41, is the first sitting MP to be killed since 1990, when Ian Gow was the last in a string of politicians to die at the hands of Northern Irish terror groups.

Jayda Fransen, deputy leader of Britain First, told UK media she was “extremely shocked” by reports of the incident and was working to determine if the alleged gunman had any links to her party.

“We were extremely shocked to see these reports and we are keen to confirm them, because of course at the moment it is hearsay,” Ms Fransen told The Guardian.

“This has just been brought to our attention. This is absolutely not the kind of behaviour we would condone.”

A man in a grey t-shirt was arrested after the attack. Photo: BBC

Ms Cox, representative for the seat of Batley and Spen, was described by many sources in the UK media as a rising star in the Labour Party, despite only recently joining the House of Commons.

Labour leader Jeremy Corbyn sent his condolences to Ms Cox’s family, describing her as “universally liked” in the UK Parliament, and saying his party was in “shock”.

“Jo was universally liked at Westminster, not just by her Labour colleagues, but across Parliament,” Mr Corbyn said in a statement.

“In the coming days, there will be questions to answer about how and why she died. But for now all our thoughts are with Jo’s husband Brendan and their two young children. They will grow up without their mum, but can be immensely proud of what she did, what she achieved and what she stood for.”

British prime minister David Cameron said Ms Cox’s death was a “tragedy”, and described her as “committed and caring”.

All parties have suspended their campaigns to convince Britons to vote either ‘Leave’ or ‘Remain’ in the EU referendum, which will be decided on 23 June.

In 2010 Stephen Timms, the Labour MP for East Ham, was stabbed twice in the stomach at a similar meeting with voters in his electorate.

And in 2000 a political staffer, Andrew Pennington, was stabbed to death with a sword during a constituency meeting for his employer, the local MP.

Watch a brief summary of Ms Cox’s parliamentary career: