The 41-year-old Batley and Spen MP and mum of two was rushed to hospital in Leeds after being shot and stabbed on the steps of Birstall library, in the heart of the quiet market town near Leeds.
Police announced just after 5pm that she had died from her injuries.
As news of Mrs Cox’s murder sent shockwaves across the country and the political community, campaigning for next week’s EU referendum was suspended.
A flood of tributes on social media and elsewhere honoured the “wonderful, beautiful, brilliant” young politician, who described herself as “mum, proud Yorkshire lass, Labour MP for Batley and Spen” on her social media account.
Mrs Cox’s husband Brendan led tributes to his wife.
In an emotional statement, Mr Cox said he and Jo’s friends would “work every moment of our lives...to fight against the hate that killed Jo”.
“Today is the beginning of a new chapter in our lives,” he said.
“More difficult, more painful, less joyful, less full of love.
“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 everyday of her life with an energy and a zest for life that would exhaust most people.
“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 also paid tribute to his party’s rising star MP, who was elected for the first time last year.
Mr 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.
“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.
“Jo was universally liked at Westminster, not just by her Labour colleagues, but across Parliament.
“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.
“We send them our deepest condolences.
“We have lost a much loved colleague, a real talent and a dedicated campaigner for social justice and peace. But they have lost a wife and a mother, and our hearts go out to them.”
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.”
Councillor Judith Blake, leader of Leeds City Council, said: “Our thoughts and deepest sympathies are with Jo’s family and friends at this truly awful time.
“This is an absolutely shocking incident that has robbed a family of a loving mother and wife who was simply doing her job, representing the people of Batley and Spen.
“This is a tragedy which cannot be allowed to undermine our democratic processes, processes that Jo was so proud to represent. Leeds as a neighbour stands united with Kirklees and our thoughts remain with Jo’s family as they try to come to terms with their dreadful loss.”
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Twitter was also awash with expressions of shock, grief and solidarity with Mrs Cox’s family.
Bishop of Leeds Nick Baines tweeted: “Jo Cox RIP. Shocking. Prayers for her family and friends. Terrible loss for the country, too.”
Conservative MP for Morley and Outwood Andrea Jenkyns said: “I am absolutely devastated to hear of Jo Cox’s death. Every MP stands united in sending our thoughts and prayers to her family.”
Mehboob Khan, leader of Kirklees council, tweeted: “I knew Jo well, she was a fantastic MP in Kirklees, who only wanted to help people in Batley & Birstall”
Jolyon Rubenstein described her as “an incredible advocate of the disenfranchised, the weak and vulnerable. A staunch advocate of many discriminated groups”.
Martin Kelner said: “Truly desperate news. Obviously thoughts with her family, friends, and colleagues.”
John Travis honoured a “fantastic campaigning MP and a lovely person”, adding “two young children are without a mum tonight”.
Andrew RT Davies said: “Killed doing her job, serving her community and her constituents. Simply dreadful.”
Stephen Naylor said: “I find it impossible to comprehend we are in a country where an MP can be murdered on a West Yorkshire street. Unbearably sad.
Sarah Whitham said: “Just so sad. MP Jo Cox was clever, funny & had brilliant if not cheesey taste in music. My heart goes out to her family, esp her children
Jawad Khan recalled a woman who “helped me to believe in myself. Now I’m going to make you proud. Our community has lost one of the kindest souls I’ve known.”