Thousands of people were expected to protest in London Saturday against Britain potentially joining air strikes against the Islamic State (IS) group in Syria.
Parliament is expected to vote on the issue next week after Prime Minister David Cameron pushed MPs to back the move in the wake of this month's Paris attacks.
The protest is organised by the Stop The War Coalition protest movement, which is also holding a string of other demonstrations around Britain.
"The proposed vote in parliament on bombing Syria by British forces is likely to take place within the next week," Stop The War Coalition said in a statement. "Yet this bombing will not stop terror attacks."
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In London, actor Mark Rylance and musician Brian Eno will also be handing a letter in to Downing Street urging Cameron not to join the international air strikes.
Britain is already involved in air strikes against the Islamic State in Iraq but has not so far taken part in bombings of the group's positions in Syria.
Cameron's previous government suffered a humiliating defeat in 2013 over military action against the Assad regime and did not push joining air strikes in Syria to a vote last year, amid resistance from the main opposition Labour party.
Current Labour leader Jeremy Corbyn, who helped set up the Stop The War Coalition is facing deep splits in his party over the looming vote.
He opposes air strikes while many of his MPs are in favour and he must decide imminently whether to let individual lawmakers vote with their consciences or try and force them to oppose it.