Parliament has been in recess ahead of this week's national vote on leaving the EU. But in a rare move, members of Parliament were recalled Monday to pay their respects in the House of Commons to the 41-year-old Labour politician, who was fatally shot and stabbed Thursday in Birstall in northern England.
British Prime Minister David Cameron said Cox, a former aid worker, had "lit up the lives of all who knew her, and saved the lives of many she never ever met."
"We express our anger at the sickening and despicable act that killed her as she did her job serving her constituents on the streets of Birstall," he said.
"But above all in this house, we pay tribute to a loving, determined, passionate and progressive politician who epitomized the best of humanity and who proved so often the power of politics to make our world a better place."
He called for lawmakers to continue her legacy "by uniting against the hatred that killed her."
Jeremy Corbyn, leader of the opposition Labour Party, called for a kinder, gentler politics in the wake of her killing.
"We have lost one of our own and our society as a whole has lost one of our very best," he said.
"Jo Cox didn't just believe in loving her neighbor, she believed in loving her neighbor's neighbor."
Roses were placed in the empty seat where Cox usually sat in Parliament.
A memorial service for the first-term politician was scheduled in St. Margaret's Church, the parish church of the House of Commons.
Colleague: We're 'still all in shock'
Labour colleague Keith Vaz, who had campaigned for Cox in her electorate of Batley and Spen, said the move was important for members of Parliament, who were "still all in shock."
"We've not had the opportunity to see colleagues," he said.
"All this is a big shock to Parliament. You simply do not expect this to happen in Britain, because we have the toughest gun laws in the world, and certainly MPs don't have the kind of security that you have in other countries."
Cox was a rising star of her party who was a vocal advocate of Britain remaining in the EU.
The man accused of killing her appeared in a London court Monday via video link.
Tommy Mair, 52, spoke only to confirm his name and was remanded into custody until Thursday. When he first appeared in court Saturday charged with murder and other offenses
, Mair told the judge: "My name is death to traitors, freedom for Britain."
Cameron: 'Stay and fight'
Both the "Leave" and "Remain" campaigns, which have been neck and neck in the polls, held rival rallies in London.
Cameron, who has led the campaign to remain, restated his case to the nation Sunday night on the BBC's "Question Time."
"Let's be clear if we do leave, that's it," Cameron said. "We are walking out the door, we're quitting and we are giving up on this organization, which, even if we leave, will have a huge effect on our lives, on our children, on our opportunities, on our businesses, and I don't think Britain in the end is a quitter -- I think we stay and fight."
Britain's Sunday Telegraph newspaper published an article Sunday by Cameron
in which he warned a vote for "Brexit" would leave Britain a "permanently poorer country in every sense" and reduce its global influence.
Boris Johnson: 'Take back control'
Boris Johnson, a member of Parliament and former London mayor who has been one of the most high-profile advocates for the "Leave" campaign, responded with a column in the newspaper
arguing the "Remain" camp offered "nothing but the steady and miserable erosion of parliamentary democracy in this country."
"If we vote Remain, we stay locked in the back of the car, driven by someone with an imperfect command of English, and going a direction we don't want to go," he wrote.
"The Remainers are now desperately trying to suggest that anyone who wants to Leave is somehow against the spirit of modern Britain; against openness, tolerance, decency. What nonsense -- and what an insult to the people of all races and parties and ages and beliefs who simply want to take back control of this country's democracy."
The British pound rallied strongly Monday
following a series of opinion polls that suggested momentum may be swinging in the direction of a "Remain" vote. Economists have warned that voting to leave the EU could cause enduring damage to Britain's economy.
But polling has consistently shown voters sharply divided on the issue, with a significant group undecided. Observers have cautioned against setting too much store in the opinion polls after they proved inaccurate in predicting the outcomes of recent votes.
Less divisive tone?
It remains to be seen how the killing of Cox -- a highly regarded lawmaker who campaigned for Britain to stay in the EU -- will affect the final days of the referendum campaign.
Campaigning to date has often been acrimonious, with inflammatory topics such as immigration at the center of the debate.
Jeremy Cliffe, a columnist at The Economist, told CNN that the campaign had been "more vitriolic, more divisive, more angry than I think anything we've seen in Britain for decades -- much more so than at a general election."
As an example of the "dangerous rhetoric," he pointed to a widely condemned poster -- depicting a column of Middle Eastern migrants walking through Eastern Europe beneath the words "Breaking Point" -- that UK Independence Party leader Nigel Farage unveiled only hours before Cox was killed Thursday.
But Cliffe said the mood since the campaign resumed had been "a lot more somber, a lot more calm."
Many pundits predict that both camps will attempt to strike a less divisive tone in the wake of Cox's death -- the first time a sitting British lawmaker has been killed since 1990.
Julia Ebner, a policy analyst at counterextremism think tank Quilliam, said the "Leave" campaign faced a dilemma in whether to persist with pushing the issue of immigration -- which has been central to many of its arguments -- in the aftermath of Cox's killing.
"I think the 'Leave' campaign will have to look for a different thing to focus on now," Ebner told CNN.
Johnson went to lengths at a rally Sunday to brand himself as "pro-immigration" and frame the "Leave" argument as a move to "take back control" of Britain.
Cox's Labour colleague Vaz said that neither side would want to try to use the death to further its cause, adding that "it would be wrong to do so."
"At the heart of this is a human tragedy," he said. "A young woman -- a mother, a wife -- who has been killed in the most brutal circumstances. No one will want to use this death in any way to justify any of the arguments."
Lord accuses campaign of xenophobia
Baroness Sayeeda Warsi, a member of the House of Lords who was a former British foreign minister and chair of the ruling Conservative Party, announced Monday she was ending her support for the "Leave" campaign, accusing it of peddling "lies, xenophobia and the politics of hate."
"Toxic, divisive (and) xenophobic political campaigning should have no place in a liberal democracy," she wrote on Twitter. "The politics of hate should always fail."
Some in the Leave camp expressed surprise at the defection, saying Warsi had never been an active member of the campaign.
"When I invited Sayeeda Warsi to join the Leave campaign, she declined," Daniel Hannan, a Conservative member of the European Party and Vote Leave campaign committee member, wrote on Twitter.
"Fair enough, obviously. But how is this a 'defection'?"
His comments were echoed by Conservative lawmaker Nadhim Zahawi, who wrote on Twitter that he had been "part of the leave campaign from start."
"I had no idea that Sayeeda Warsi was part of the leave campaign. News to all of us me thinks."