By William James

LONDON (Reuters) - The widower of murdered British lawmaker Jo Cox said her killing was an act of terror as he paid tribute to his late wife on Wednesday alongside hundreds of mourners at an emotional memorial in London's Trafalgar Square.

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."

The vigil marking what would have been her 42nd birthday, and also attended by Nobel laureate Malala Yousafzai, fell silent at 1518 GMT (1118 EDT) in a mark of respect also observed at other events across the country and around the world including in Beirut, Brussels, Melbourne, Nairobi, Geneva, 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).

In Geneva, friends remembered her at a sunny lakeside vigil outside the Centre for Humanitarian Dialogue where U.N. human rights chief Zeid Ra’ad al Hussein was among those paying tribute.

"Jo was very much a member of this community ... the very things that she shood for, she campaigned for, she poured her heart and soul into, was the very agenda that we all signed up for," Zeid told Reuters.

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

(Additional reporting by Tom Miles; Editing by Stephen Addison)