British Prime Minister David Cameron canceled a planned visit to Gibraltar as part of his campaign for Britain to remain in the EU.

"We have lost a great star. She was a great campaigning member of parliament with huge compassion, with a big heart," Cameron said in a statement.

"It is 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," he added.

Labour Party leader Jeremy Corbyn said: "Jo Cox died doing her job at the heart of our democracy, listening to and serving the people she was elected to serve. The whole country is in shock and grief at the horrific murder."

"The incident is terrible, dramatic and our thoughts are with the people affected -- the Labour lawmakers, the politicians. I don't want to connect this with the vote on Great Britain staying in the European Union," German Chancellor Angela Merkel said.

French Prime Minister Manuel Valls said: "Deeply sad for Jo Cox and the British people. Through her it's our democratic ideals that were targeted. Never accept that!"

U.S. Secretary of State John Kerry also expressed his sorrow, calling the attack "an assault on everybody who cares about and has faith in democracy. And our thoughts are profoundly with the family -- her husband, her children -- and with all of the British people, who I know feel the loss profoundly."

"My thoughts are with her family, her friends, and the British people. It was a true shock to me that a British politician was killed during the campaign," Danish Prime Minister Lars Rasmussen said.