Cox, a supporter of Britain staying in the EU, was shot and repeatedly stabbed in her own constituency near Leeds in northern England by a man who witnesses said had shouted "Britain first".
She was pronounced dead just over 48 minutes later by a doctor working with a paramedic crew trying to save her life. A 52-year-old man was arrested by officers nearby and weapons including a firearm were recovered.
The killing prompted a halt to campaigning in the EU referendum. Though the motives of the killer were not immediately clear, some speculated that sympathy for Cox could boost the Remain campaign which has fallen behind Leave in recent days.
Police said they were not in a position to discuss the motive of the attack.
"Jo believed in a better world and she fought for it every day of her life with an energy and a zest for life that would exhaust most people," Cox's husband, Brendan, said.
"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."
Britain's Union Jack flag was flying at half-mast over the Houses of Parliament, Downing Street and Buckingham Palace, while in Birstall hundreds of people attended a vigil at a local church.
Some people, many weeping, laid flowers outside the Houses of Parliament. Others threw flowers onto the houseboat on the Thames where Cox had lived with her husband and young children.
British politicians paid tribute to Cox and expressed shock at the killing, as did leaders across Europe and the world.
Prime Minister David Cameron said the killing of Cox, who had worked on U.S. President Barack Obama's 2008 election campaign, was a tragedy.
"We have lost a great star," said Cameron, who called the referendum. "She was a great campaigning MP with huge compassion, with a big heart. It is dreadful, dreadful news."
Hillary Clinton said she was horrified. German Chancellor Angela Merkel called the attack "terrible" but added that she didn't want to link it to the EU referendum.
Who killed Cox?
Media reports, citing witnesses, said the attacker had shouted out "Britain first", which is the name of a right-wing nationalist group that describes itself on its website as "a patriotic political party and street defense organization".
The deputy leader of the group, Jayda Fransen, distanced it from the attack, which she described as "absolutely disgusting".
West Yorkshire's elected Police and Crime Commissioner said "our information is that this is a localized incident, albeit one that has a much wider impact".
The killer was named by media as Thomas Mair.
Family members, including his brother, said that Mair had not expressed strong political views, the Guardian newspaper reported.
"He has a history of mental illness but he has had help," the Guardian quoted his brother, Scott Mair, as saying.
"I am struggling to believe what has happened. My brother is not violent and is not all that political. I don't even know who he votes for."
Neighbors were quoted by media as describing a man who had lived in the same house for at least 40 years and helped locals weed their flowerbeds.
Gun ownership is highly restricted in Britain, and attacks of any nature on public figures are rare. The last British lawmaker to have been killed in an attack was Ian Gow, who died after a bomb planted by the Irish Republican Army (IRA) exploded under his car at his home in southern England in 1990.
Colleagues expressed their shock and disbelief at the death of Cox, a Cambridge University graduate who had spent a decade working for aid agency Oxfam and was known for her work on women's issues.
"We've lost a wonderful woman, we've lost a wonderful member of parliament, but our democracy will go on," Labour leader Jeremy Corbyn said in a televised statement. "As we mourn her memory, we'll work in her memory to achieve that better world she spent her life trying to achieve."