Jeremy Corbyn has launched a staunch defence of the Stop the War Coalition in the face of criticism from senior figures in his own party.
The defiant Labour leader has insisted the protest group is one of the most important campaigning organisations of modern times and is a "vital force at the heart of our democracy".
Mr Corbyn, who chaired the movement before taking charge of the party in September, attended its fundraising dinner in London despite calls from a number of MPs, including former frontbenchers Tristram Hunt and Caroline Flint, to shun the event in the wake of a series of controversial statements about terrorism and air strikes on Syria.
A tweet and article published following the attacks on Paris suggested France had ''reaped the whirlwind'' of Western support for extremist violence in the Middle East.
Mr Corbyn entered the Turkish restaurant in Southwark by the back door, avoiding most of the photographers and television cameras awaiting his arrival.
He told guests at the dinner: "The Stop the War Coalition has been one of the most important democratic campaigns of modern times.
"It has brought hundreds of thousands of people on to the streets time and again. It has organised protests and lobbies in every part of the country, including by military families.
"Most of all, 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.
"The anti-war movement has been a vital force at the heart of our democracy. Branding it as somehow illegitimate is an attempt to close down democratic debate and campaigning."
Stop the War chairman Andrew Murray said it was "ridiculous" to suggest that Mr Corbyn should have stayed away from the event.
He told Sky News: "First of all, ridiculous that when Britain has voted for war, a life-and-death matter literally, people are talking about a Christmas fundraising party.
"Secondly, it's absurd because Jeremy Corbyn helped found the Stop the War Coalition, he has campaigned with us for 14 years now. Why on earth should he not celebrate Christmas with his closest friends and his strongest supporters?"
Green MP Caroline Lucas earlier this week quit as a patron of Stop the War, citing concerns about the positions it has adopted.
Lindsey German, a founding member of the group, said she did not believe the resignation was "necessary".
The activist insisted that the group was not an "apologist" for Bashar Assad and dismissed suggestions the group should campaign against the Syrian dictator and other brutal regimes rather than focusing only on Western action.
She told Channel 4 News: "Isis came from the occupation of Iraq. That's where it started and that's how it got into Syria, so let's not pretend this has got nothing to do with Western intervention."