MPs across the political divide reacted with horror after Labour MP Jo Cox was shot and killed in Birstall town centre.

The married mum of two young children was attacked after she held an advice surgery yesterday for her Batley and Spen constituents.

Tommy Mair, 52, was detained by police near his home in Birstall, and was still being held this morning.

Downing Street said flags across Whitehall would be lowered to half mast.

Prime Minister David Cameron said: “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.”

He added in a statement: “This is absolutely tragic and dreadful news and my thoughts are with Jo’s husband, Brendan, their two children and wider family. We’ve lost a great star. She had a big heart and people are going to be very, very sad at what has happened.

“She was a very strong campaigning MP. She had a great track record of caring about refugees and had taken a big interest in how we can look after Syrian refugees and do the right thing in our world. She was a star for her constituents, a star in Parliament and a star right across the House.

“It’s right that we are suspending campaigning activity in this referendum and everyone’s thoughts will be with Jo’s family and her constituents at this terrible time.”

Labour leader Jeremy Corbyn said the country was in shock at her killing.

“Jo had a lifelong record of public service and a deep commitment to humanity. She worked both for Oxfam and the anti-slavery charity, the Freedom Fund, before she was elected last year as MP for Batley and Spen – where she was born and grew up.

“Jo was dedicated to getting us to live up to our promises to support the developing world and strengthen human rights – and she brought those values and principles with her when she became an MP.

“Jo died doing her public duty at the heart of our democracy, listening to and representing the people she was elected to serve. It is a profoundly important cause for us all.”

Theresa May described Ms Cox as one of the “brightest and most popular” MPs and the pain her family and friends were suffering as “unimaginable”.

The Home Secretary added: “Our thoughts and prayers are with them at this terrible time.

“The full details of what happened outside Jo’s constituency surgery in Birstall this afternoon are not yet clear and until the facts have been fully established I will not comment further.

“It is entirely appropriate that all campaigning for the referendum has been suspended. All of us are united in our deep sadness at the loss of one of our brightest and most popular Westminster colleagues.”

Chancellor George Osborne said: “Incredibly sad to hear shocking news about Jo Cox. All my thoughts with Brendan and their children.”

And Foreign Secretary Philip Hammond said he was “deeply saddened by news of the death of Jo Cox”.

Former Labour MP Joan Walley, in whose parliamentary office Ms Cox had worked before becoming an MP herself, was being interviewed on BBC Radio 4’s PM programme as news of her death 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.

“Words can’t express what I feel because Jo’s life and everything she believed in are so badly needed now. My thoughts and prayers are with her family and I just hope all Jo’s many family, friends, admirers, whatever political side they are, will unite to think about her, to think about our parliamentary democracy and to think what we can do together to be true to the things she believed in.”

Former Scottish First Minister Alex Salmond said: “Absolutely devastating news about Jo Cox. Truly dreadful. One of the brightest and best of the new MPs and a great loss to the country.”

Lord (William) Hague said: “Terribly sad to hear of the death of Jo Cox, senselessly killed working for her constituents.”

Former Cabinet minister Iain Duncan Smith said: “I am shocked and saddened to hear that Jo Cox has died as the result of this brutal attack. My prayers are with her family at this terrible time.”

Former international development secretary Andrew Mitchell told PM: “My thoughts – as everyone across Parliament’s will be – are with her husband Brendan and her two lovely little children, who used to come into Portcullis House to have tea with her during the week.

“It is an unspeakable and un-understandable event that has taken place, and Parliament and her constituents will be much the poorer for her passing. But it’s her family and those two little children who we must be thinking about now.”

Labour MP Jess Phillips said: “My heart is broken. She was amazing, a brave giant who stood against bullies.”

Foreign Secretary Philip Hammond said: “Deeply saddened by news of the death of Jo Cox.”

Scotland’s First Minister Nicola Sturgeon said: “I didn’t know Jo Cox but by all accounts she was a brilliant young MP. And today she was just doing her job. My heart breaks for her family.”

Liberal Democrat leader Tim Farron said: “I am utterly heartbroken at the news of the death of Jo Cox. This is a devastating attack on our peaceful society.”

Scottish Labour leader Kezia Dugdale said: “There will be time to talk about what the savage murder of an elected representative means for our democracy but that’s for another day. Today we mourn the loss of one of our own.

“Jo was an extraordinary woman. She devoted her life to helping people in the darkest places in our world. Our hearts are broken by the loss of one of our country’s brightest hopes. We mourn not just the woman she was but the loss of everything she would have achieved.

“Our thoughts and love are with her husband Brendan and her children. That their pain and loss is shared by people across the Labour family and people around the world is testament to how special the life Jo led was. In time we hope that will be some comfort to them.”

Labour’s Yvette Cooper said Ms Cox had been “one of liveliest, bravest and most passionate MPs”, while her husband Ed Balls said there were “no words to describe how awful this is”.

Former Prime Minister Gordon Brown said: “Our memories will be forever scarred by this moment. Our hearts will always be hurt at our country’s loss. Jo Cox was the most vivacious, personable, dynamic and committed friend you could ever have. Whenever you talked to her, the compassion in her eyes and the commitment in her soul shone through. Years of public service advancing the causes she cared passionately about should have been ahead of her.

“Sarah and I were privileged to work with Jo and her husband Brendan over many years and in her tireless efforts on behalf of poor and desolate children and mothers. She went to some of the most dangerous places in the world. The last place she should have been in danger was in her home town.

“People will say that this does not happen in Britain. This should not happen in Britain. And we must resolve that this will never again happen in Britain.”

Boris Johnson said: “My thoughts are with Jo and her family. Appalling an MP should lose her life simply doing her best for constituents.”