Labour leader Jeremy Corbyn has addressed a fundraising dinner for the Stop the War coalition, despite calls from some of his MPs not to attend.
Mr Corbyn said the group was a "vital force" and "one of the most important democratic campaigns of modern times", which had brought "hundreds of thousands of people" to protests.
Campaigner Andrew Murray is replacing Mr Corbyn as Stop the War chairman.
He described the Labour leader as part of the campaign's backbone.
Mr Murray, a trade unionist and journalist who previously chaired Stop the War for 10 years, said the quarter of a million people who had voted for Mr Corbyn to lead Labour "did so because of his stand against war, not in spite of it".
He said: "This unity between a mass campaigning movement and the leadership of the Labour Party clearly makes some uncomfortable.
"Their new year's resolution needs to be to get used to it."
Mr Corbyn, who has been a leading member of the Stop the War coalition since it was founded, used his speech to praise its campaigns against military interventions.
"It has been shown to be right in opposing more than a decade of disastrous wars - in Afghanistan, Iraq and Libya - while many of its most vociferous critics supported them," he said.
Senior Labour figures, including MPs Caroline Flint and Tristram Hunt, had urged Mr Corbyn to shun Friday's event over a controversial blog Stop the War posted after the Paris attacks.
Green MP Caroline Lucas has also resigned as a patron of Stop the War, citing concerns about the positions it has adopted.
A tweet and article published in the wake of the attacks on Paris last month suggested France had "reaped the whirlwind" of Western support for extremist violence in the Middle East.
Both were later removed and disowned by the organisation's leaders, but critics have highlighted another article last week which argued jihadists were driven by a "spirit of internationalism and solidarity" akin to the International Brigades that fought in the Spanish Civil War.
'Long-standing association'
Former Labour leader Ed Miliband told BBC Newsnight he would not "commentate on what organisations Jeremy Corbyn chooses to be member of".
"He's got a long-standing association with this organisation, he's got a long-standing opposition to different types of intervention," said Mr Miliband. "He spoke on this in the Syria debate.
"I think our party's focus should be on taking the fight to the Tories and working out the ideas that are going to win us the next general election, not Jeremy Corbyn's political engagements."
Shadow culture secretary Michael Dugher said the Labour leader should use Friday's dinner to "have a word" with the organisers.
In an interview with The House magazine, Mr Dugher said many Stop the War activists thought "the wrong people won the Cold War" and that the dinner was a chance to deliver a message.
