Cox died after she was shot and stabbed in the street in her constituency in northern England last week - an attack that shocked the country and prompted an outpouring of sympathy around the globe for the lifelong humanitarian campaigner.

"Jo's killing was political. It was an act of terror designed to advance an agenda of hatred towards others," Brendan Cox said in a tearful speech to a large crowd packed into the London landmark.

"What a beautiful irony it is that an act designed to advance hatred has instead generated such an outpouring of love."

What a beautiful irony it is that an act designed to advance hatred has instead generated such an outpouring of love

The vigil marking what would have been her 42nd birthday, and also attended by Nobel laureate Malala Yousafzai, fell silent at 1518 GMT in a mark of respect also observed at other events across the country and around the world including in Beirut, Brussels, Melbourne, Nairobi, New York and Washington.

Cox's death has abruptly changed the tone of the final days of caustic campaign around a referendum on Britain's European Union membership, which takes place on Thursday, prompting a groundswell of support for a more positive campaign.

"She feared the consequences of Europe dividing again, hated the idea of building walls, and worried about the dynamics that that could unleash," Brendan Cox said.

The memorial included an offering of flowers by religious leaders of different faiths, music from the band that played at her wedding, and video tributes by rock star Bono and Cox's sister in the lawmaker's constituency.

White roses, the flower of Cox's home region, Yorkshire, floated in the square's halted fountains.

Cox's husband and their young son and daughter, whose London home was a houseboat, earlier waved to clapping crowds as they arrived by boat after mooring a dinghy filled with flowers in the River Thames outside the Houses of Parliament.

Tributes from friends, family and former colleagues painted a picture of a hard-working, loving mother who had dedicated her life to women's issues, the plight of refugees fleeing war in Syria and most recently to those she represented in parliament.

Before her election to parliament in May 2015, Cox had spent almost a decade working for aid agency Oxfam. A fund set up in her memory has so far raised 1.3 million pounds ($1.91 million).

The money will be split between The Royal Voluntary Service, which helps improve life for older people, HOPE not Hate, which combats extremism in local communities and The White Helmets, a network of rescue workers who operate in areas of Syria where medical infrastructure has broken down.

"Jo's life is a proof that a message of peace is more powerful than any weapon of war," said Malala, who in 2014 became the youngest ever winner of the Nobel Peace Prize for her work promoting women's access to education.

"Once again the extremists have failed."