UK police have reported that Jo Cox, a member of the British parliament, passed away after being shot in northern England, Al Jazeera reports.
West Yorkshire Police said on Thursday a 52-year-old man had been arrested following the incident, which prompted the suspension of campaigning for next week's referendum on the country's EU membership.
The attack took place in Birstall, near the city of Leeds. It happened outside the library where she regularly held meetings with her constituents.
"Jo was attacked by a man who inflicted serious and sadly, ultimately fatal injuries," Dee Collins, acting chief constable of West Yorkshire Police, told reporters.
Police said a 77-year-old man was also assaulted in the incident and suffered injuries that were not life-threatening.
One witness told the Press Association that Cox had intervened in a scuffle between two men, one of whom pulled a gun from a bag and then fired twice.
"She was taken by air ambulance from Birstall library near Leeds. Witnesses say she was shot and stabbed and taken by air ambulance to Leeds General Infirmary," Al Jazeera's Barnaby Phillips, reporting from London, said.
Officers said weapons, including a firearm, had been recovered from the scene.
"This was a localized incident, albeit one which has a wider impact," West Yorkshire Police and Crime Commissioner Mark Burns-Williamson told reporters.
The news of Cox's death caused deep shock across Britain.
"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," Labour leader Jeremy Corbyn said.
Prime Minister David Cameron said the killing of Cox, who was a mother of two, was a tragedy.
"We have lost a great star," Cameron said in a statement. "She was a great campaigning MP with huge compassion, with a big heart. It is dreadful, dreadful news."
Cox was a leading campaigner for Britain to remain in the 28-member bloc and chaired the all-party parliamentary committee on Syria.
A University of Cambridge graduate, she was the Oxfam aid agency's policy chief before entering parliament and a prominent campaigner for refugee rights.
She became a Labour MP for Batley and Spen in the May 2015 general election and was also known for her work on women's issues.
Following the attack, both sides in Britain's June 23 referendum on leaving or staying within the EU said they were suspending campaigning, while Cameron pulled out of a planned rally in Gibraltar.
The Stronger in Europe camp said it was "suspending all campaigning" for Thursday and Friday, while a spokesman for the rival Vote Leave group, which is backing a so-called Brexit, said that their "battle bus" was returning to headquarters.
Britain's flag was flying at half-mast over the Houses of Parliament in London, while in Birstall hundreds of people attended a vigil at a local church.