Image source, Getty Images

Labour's Yvette Cooper embraces the Bishop of Leeds, Nick Baines, following a vigil at St Peter's Church in Birstall, West Yorkshire for the MP Jo Cox, who died after being attacked in the street there earlier today.

The attack on Jo Cox was an assault on the values of "freedom, liberty and justice", Chancellor George Osborne has said in an emotional speech at Mansion House, in the City of London.

He said MPs' accessibility to constituents was a virtue of the UK's political system.

Sarah Brown, wife of former prime minister Gordon Brown, worked alongside Jo Cox on a global campaign on maternal mortality. 

Despite the serious nature of the cause, she said the progress they made in changing governments' policies and budgets to save mothers' and children's lives made for a "positive experience". 

And she spoke of Jo Cox's roots, her husband and two young children. 

Image source, Reuters

The death of Labour MP Jo Cox, attacked outside her constituency surgery, is the latest in a series of acts of violence against MPs.

A report out earlier this year by psychiatrists working with the Home Office, reported at the time in the Guardian, external, suggested four out of five MPs had suffered intrusive or aggressive behaviour and some feared going out in public.

While MPs are protected by high security measures in Parliament, it is a different matter in their constituencies.

But can more be done to improve their security? Read more

Shadow Education Secretary Lucy Powell said the loss of her friend and colleague of 20 years, Jo Cox, was "absolutely devastating".

Former Director of Public Prosecutions, now Labour MP Keir Starmer said Jo Cox's colleagues were finding it difficult to come to terms with the idea that somebody "defined by love and compassion should be killed in this way". 

Talking about MPs' security, he said it was "important" constituents had access to MPs but that meant there were security risks. 

Labour leader Jeremy Corbyn said the party was "devastated" by Jo Cox's killing. 

The ability of MPs and their staff to engage with constituents goes to the heart of our parliamentary democracy. 

MPs can represent us here at Westminster only because every week they go back home and meet us face to face. 

There will inevitably be another debate about MPs' security. Many are often subject to physical attack. 

But politics can't happen from behind a ring of steel. Jo Cox's murder is not just an awful tragedy. It is also an assault on our democracy.

Read more from James

Gabby Giffords, the former US congresswoman who suffered a severe brain injury after being shot in the head in 2011, has tweeted her reaction to Jo Cox's death.

Speaking in Copenhagen, the US Secretary of State John Kerry described the killing of Jo Cox as an assault on democracy.  

The Speaker of the House of Commons, John Bercow MP, has made a statement following the death of Jo Cox, Labour MP for Batley and Spen.

Laura Kuenssberg
BBC political editor

Image source, PA

It's an over-used word these days, but Jo was a star. She was brave, funny, and clever.

In Westminster she was one of those rare people who, whatever was going on in politics that day, however much backbiting there was, however many insults were being thrown around, the day was always improved if you bumped into her for a chat, a cup of tea, or heard her speak fearlessly and with compassion in the Commons.

But a conversation with her was just as likely to turn to her young kids, or the family's houseboat on the Thames, as it was to talk about the latest political machinations.

Jo was an MP who cared deeply about her party, and how politics can make a difference not just here, but around the world.

But more importantly she was a mum, a wife and a daughter who broke into ready smiles when she talked about her family. That's what I'll remember.

Read more

Len McCluskey, leader of the Unite union, said Jo Cox died "doing what she believes in and was passionate about".

Image source, PA

Brendan Cox, the husband of Labour MP Jo Cox,  said: 

Nick Robinson
Presenter, Radio 4 Today

People like me are paid to find the right words. Words to describe, to analyse, to explain. Words that match the moment.

Today is one of those days when it is very hard to find the words which convey the shock that I - that all those who are in politics or report on it - feel at the news that an MP has been murdered on the streets of Britain.

Jo Cox - a popular and respected Labour member of parliament, a young mother and a campaigner for the rights of refugees - was shot and stabbed by a man outside her constituency surgery. She was vulnerable to attack because she was, like so many MPs, available to anyone who wanted to see her with any grievance.

Her death is a reminder that our elected representatives, who are so often demonised for living separate lives from the rest of us, actually all too often live in our communities, in our streets worrying about the same things that we do.  Unlike us, though, they open themselves up not just to criticism and abuse but to assault by those who disagree with them.

We do not know the exact circumstances of this attack. The police insist that it was "localised" - ie not linked to any global terror threat. There are, though, suggestions that it was political. If that turns out to be true it is a reminder of one other thing too. The need at all times to respect those who disagree with us and to understand that anger, fury and rage are not the same as passion or belief.

It is fitting that today the cacophony of this referendum campaign is briefly on hold.

Former Labour MP Joan Walley was live on BBC Radio 4's PM programme when the death of Jo Cox was announced.

Scottish Labour leader Kezia Dugdale has hailed Jo Cox as "an extraordinary woman" who "devoted her life to helping people in the darkest places in our world".