Former Labour leader Ed Miliband has appealed for the party to focus on fighting the Tories as the party continues to tear itself apart under his successor.
Jeremy Corbyn ignored calls by senior figures to stay away from a Stop the War Coalition dinner in the wake of a series of controversial statements it made about terrorism and air strikes on Syria.
The party leader instead defiantly insisted the protest group is one of the most important campaigning organisations of modern times and described it as a "vital force at the heart of our democracy".
Mr Miliband refused to condemn Mr Corbyn's decision to attend the event but urged the party to focus on fighting the Conservatives.
He told BBC Two's Newsnight: "I'm not going to commentate on what organisations Jeremy Corbyn chooses to be a member of. He's got a long-standing association with this organisation, he's got a long-standing opposition to different types of intervention. 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."
Mr Corbyn, who chaired the Stop the War Coalition before taking charge of Labour 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 after 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.
The party leader entered the Turkish restaurant in Southwark by the back door on Friday evening, avoiding most of the photographers and television cameras awaiting his arrival.
Mr Corbyn's defence of the movement comes as it prepares to stage another protest against military intervention in Syria, with thousands expected to join the march today.
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?"