Jeremy Corbyn has led the tributes to Jo Cox, describing the Labour MP as a "wonderful woman and a wonderful MP".
Police today confirmed the 41-year-old mother-of-two has been killed after being shot and stabbed in Birstall, near Leeds.
The tragedy prompted an outpouring of sympathy from across the political spectrum, with Sadiq Khan, the Mayor of London, calling her one of the "smartest and most independently minded MPs" and "a fearless campaigner".
David Cameron said the MP for Batley and Spen was a "star for her constituents, a star in Parliament and a star right across the House".
The former aid worker in conflict zones was elected last year and campaigned fiercely for Britain to provide sanctuary for refugees during her time in Parliament.
An emotional Corbyn said the "whole of the Labour family is devastated tonight". See video above. He went on:
"Jo Cox has been killed doing her duty, doing her job as a constituency MP. She is somebody who dedicated her life to human rights and to justice. She worked for anti-slavery campaigns, she worked for Oxfam, and she became an MP for the place where she grew up and loved.
"In Parliament, she was respected across the whole House. She was somebody with a deep commitment and deep compassion for human rights and justice. Sadly today she died. She was killed on the streets of her constituency doing her duty, as a local member of Parliament helping people.
"She leaves behind a husband, a wonderful man who likewise has spent his life campaigning for justice. She leaves behind two young children who will grow up never to see their mum again. She can be proud of what she was, she can be proud of what she did. And they can be very proud of everything she stood for."
In further tributes, Sadiq Khan wrote:
"Everyone who met Jo knew she was special. I knew her from her time as a fearless campaigner working on behalf of some of the world’s poorest and most marginalised people and helped her, a little, in her campaign to become the Labour candidate and MP for Batley and Spen. I knew that she would bring all her passion for social justice and equality to Parliament and fight just as hard for her own community in Westminster as she has for so many others around the world.
"In the year she was an MP she made more impact than others make in a whole parliamentary career. Jo loved being the Member of Parliament for Batley and Spen. She was the most powerful advocate for the people of Syria, ensuring their plight stayed on the political agenda as well as reminding us all of our country’s proud record of humanitarianism and brought her expertise to bear on some of the most pressing global challenges of our time. But she was also an extremely hard-working local MP for her home constituency and so proud of her roots there.
"Jo was one of the smartest and most independently minded MPs, respected by people from all parties. She was also warm and funny – I remember her telling me recently that she was going to get permission from the House of Commons to park her little boat at Parliament so she could drive it to work.
"It is typical of Jo that she was serving her community today, doing what she loved, when this horrific attack happened. She was the best of politics, the best of Labour and I will miss her. All my thoughts and prayers are with her husband Brendan, her two little children, and her whole family."
Prime Minister David Cameron said:
“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.”