The Rev. Paul Knight, who led the service, told CNN that those attending "rejoiced in what Jo was able to accomplish in her short time in Parliament."
"She was a shining example of the way we ... should work together," he said.
He delivered a sermon on the parable of the Good Samaritan as an appropriate example of how people should help others, "whatever their creed," as Cox -- a former aid worker who advocated for Syrian refugees and other causes -- had done.
Cox was a vocal supporter of Britain remaining in the EU. When the suspect in her killing
, 52-year-old Tommy Mair, appeared in court charged with murder and other offenses Saturday, he said, "my name is death to traitors, freedom for Britain."
Rival rallies in London
Cox's killing shocked the nation and brought a temporary halt to the bitter campaigning that has taken place for months leading up to Thursday's national referendum.
Both the "Leave" and "Remain" campaigns, which have been neck-and-neck in the polls, resumed their official activities Sunday with rival rallies in London.
At a "Leave" event at Old Billingsgate, MP and leading "Brexit" advocate Boris Johnson told attendees that Britain was "at its best when we believe in ourselves."
"This is our moment," he said to cheers.
"Let us go forward with quiet, polite determination over the next four days and let's do this thing. Let's do it together. Let's take back control."
At a rally across town in Hyde Park, "Remain" supporters arranged themselves to spell out the word "In" for an aerial photograph. A minute's silence in tribute to Cox was observed beforehand.
Hopes for less 'toxic' campaigning
With passions running high over such a momentous decision affecting the strategic future of the country, the debate leading up to Thursday's referendum has often been rancorous.
Figures on both sides have been accused of lying and making up their arguments, and the debate has repeatedly centered around politically incendiary issues such as immigration.
But observers are predicting that both sides will attempt to strike a more measured tone as they make their case following Cox's killing.
Speaking Sunday, Johnson went to lengths to describe himself as "pro-immigration," saying he was a descendant of Turkish immigrants, and he backed an amnesty for illegal immigrants who had been in the country for 12 years. Rather than focusing on the perceived threat of rising immigration, he framed the "Leave" argument as a move to "take back control" of Britain.
Joe Mulhall, research editor for anti-extremist advocacy group Hope Not Hate, told CNN that while he hoped the debate would become less "toxic" in the days leading up to the referendum, he did not have "complete confidence in that."
"Every time we thought it had got as bad as it could have, it's gone and got worse," he said.
Cox's seat in Parliament
Major political parties have publicly announced they will not contest the late politician's seat in the by-election out of respect, but one former British National Party member has ruffled feathers with his announcement to take on Cox's seat.
Jack Buckby, the press officer for the Liberty GB Party, posted a video on social media Saturday saying he and his party are going to run.
"Because the Labor Party must be held to account, we cannot allow the Labor Party to completely walk this election completely uncontested," Buckby said in the YouTube video.
The Liberty GB Party proclaims to be "a patriotic, anti-Islamisation party promoting Christian civilisation, Western freedoms and British culture" on their Facebook page
The Conservative Party stood firm earlier this week that they will not run a candidate in the election to replace Cox.
"Following the tragic killing of Labour MP Jo Cox, the Conservative Party has decided not to contest the forthcoming by-election as a mark of respect to a much-loved and respected politician," the Conservative Party said in a statement earlier in the week.
PM: Brexit would leave Britain 'permanently poorer'
British Prime Minister David Cameron, who has led the campaign to remain, wrote an article published in the UK's Sunday Telegraph newspaper
in which he warned that a vote for "Brexit" would leave Britain a "permanently poorer country in every sense" and reduce its global influence.
"It would be a one-off and permanent diminution in our standing in the world; an abject and self-imposed humiliation for a proud and important country like ours," wrote Cameron, whose "Remain" campaign is facing its most effective opposition from members of his own Conservative party.
"That's why all of our major allies want us to remain; only our adversaries want us to leave."
Paying tribute to Cox in the article, he acknowledged that it was hard to turn to the question of the referendum in the wake of her killing, but wrote that Britons would rarely face such a momentous decision. "And here's the thing: it's irreversible. There is no turning back if we leave."
Britain's newspapers are proving as divided on the EU referendum issue as its public and political class. The Sunday Telegraph, the publication which ran Cameron's column, came out with an editorial column in favor of Brexit
, joining The Sunday Times and tabloid newspaper The Sun in the "Leave" camp.
Other British newspapers including The Times, the Mail on Sunday
, The Observer
and the Financial Times have run editorials backing a "Remain" vote.
Sister: 'She will live on'
The services Sunday in Birstall are the latest in a series of tributes to Cox, who was raised in the nearby town of Heckmondwike.
Cox's sister, Kim Leadbeater, made an emotional speech at a memorial in Birstall Saturday, telling those assembled that "our family is broken, but we will mend over time."
"We will never let Jo leave our lives," she said. "She will live on through all the good people in the world."
On Sunday, Cox's husband, Brendan Cox, tweeted that he had spent the night camping with their children to remember her.
"Jo loved camping," he wrote. "Last night the kids (and) I camped in her memory (and) remembered the last time we were all woken by the dawn chorus."