Jo Cox, a British member of parliament who campaigned for the country to stay in the EU and for more aid for Syrian refugees, was killed in a brutal attack on Thursday.
A 52-year-old man is in police custody after Cox was reportedly shot and stabbed numerous times in her constituency in northern England.
Campaigning for the June 23 referendum was suspended out of respect, a suspension that will extend to Saturday.
This is what we know so far:
Who was Jo Cox?
The 41-year-old mother of two children was elected last year as an opposition Labour party MP for Batley and Spen, a constituency in West Yorkshire, where she grew up.
After graduating from the University of Cambridge, she helped set up a pressure group, Britain in Europe, and spent two years working in Brussels.
She was policy chief for aid agency Oxfam and after entering parliament, was a prominent campaigner for refugee rights.
Cox co-chaired the parliamentary group on Syria and spoke out in favour of immigration in a powerful first speech in the House of Commons.
What happened?
The attack took place in Birstall, a large village in Yorkshire, outside her regular constituency advice surgery in a local library.
Police said they were called at 12:53 pm (1153 GMT). Her meeting was scheduled for 1:00 pm, according to her website.
She was "attacked by a man who inflicted serious, sadly ultimately fatal, injuries", police said. She was pronounced dead at 1:48 pm.
Eyewitnesses cited by British media said she had been shot two or three times before being stabbed repeatedly.
The father of one of Cox's aides, Fazila Aswat, who was at the scene, said she had told him that the attacker seemed to have been waiting for the MP to drive up.
"The car stopped and Jo decided to come out. My daughter didn't know she'd been shot. Because this person must be waiting outside where the surgery happens," Gulham Maniyar told ITV news.
There were reports that the suspect shouted "Britain first" or "put Britain first", although this is disputed.
There was a further attack on a 77-year-old nearby who sustained injuries that were not life threatening.
"Shortly afterwards, a man was arrested nearby by uniform police officers. Weapons including a firearm have also been recovered," police said.
Who has been arrested?
British media named the suspect as Thomas Mair, described by those who knew him locally as a loner with mental health problems, including obsessive compulsive disorder.
The Southern Poverty Law Center, a US advocacy group, said it had records showing Mair had bought copious reading material from the National Alliance, a neo-Nazi organisation.
The Guardian said police were believed to have found Nazi regalia and far-right literature in their search of his house.
The Daily Telegraph newspaper reported that Mair was a subscriber to S. A. Patriot, a South African magazine published by the pro-apartheid group the White Rhino Club.
Who were Cox's family?
She was married to Brendan Cox, a former adviser to prime minister Gordon Brown who also worked at the Save the Children charity.
They had two young children, Lejla and Cuillin, and lived on a converted barge near Tower Bridge in London.
Her husband 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."
Who else has reacted?
Conservative Prime Minister David Cameron said the nation was "shocked" by the murder.
On a visit to Birstall Friday, he said: "Where we see hatred, where we find division, where we see intolerance we must drive it out of our politics and out of our public life and out of our communities."
Standing alongside him, Labour leader Jeremy Corbyn described Cox as "an exceptional, wonderful, very talented woman".
US Secretary of State John Kerry expressed his "deep sorrow" for Cox's death, adding: "It is an assault on everybody who cares about and has faith in democracy."
How rare are MP killings?
Labour MP Stephen Timms was stabbed in the stomach in 2010 by an Islamic extremist but survived the attack.
The Irish Republican Army paramilitary group killed Ian Gow in a car bomb in 1990, Anthony Berry in the 1984 Brighton bombing and shot dead Robert Bradford in 1981.
Former Northern Ireland secretary Airey Neave was killed in a car bomb in 1979 that was claimed by the Irish National Liberation Army.