Thousands of protesters gathered in central London on Tuesday in an effort to stop Britain joining air strikes on Islamic State targets in Syria, a day before a parliamentary vote on the move.
A crowd of around 4,000 marched from parliament to the headquarters of the ruling Conservative party and main opposition Labour party nearby, in the second major London protest on the issue in four days.
"We're here to say one simple thing: 'Don't bomb Syria. Don't do what you did in 2001, 2003 and 2011'," the Stop the War Coalition protest movement's Lindsey German told the crowd, referring to British involvement in wars in Afghanistan, Iraq and Libya.
"Don't go and bomb a country where me make the war even worse."
Parliament looks set to vote in favour of joining the bombing campaign against Islamic State (IS) jihadists in Syria on Wednesday, paving the way for sorties by British jets to start within days.
The debate has become increasingly heated, with Prime Minister David Cameron labelling Labour leader Jeremy Corbyn and his allies "a bunch of terrorist sympathisers" for opposing the strikes, British media reported.
In an opinion piece in Britain's Guardian newspaper late on Tuesday, Corbyn accused Cameron of having a "bomb first, talk later" strategy.
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The prime minister insists military action is needed to prevent attacks in Britain like those in Paris last month, when jihadists killed 130 people, but many experts, lawmakers and members of the public remain sceptical.
A poll in Britain's The Times newspaper by pollster YouGov to be published on Wednesday found that less than half of Britons, at 48 percent, supported Syria strikes, compared to 59 percent last week.
"It will kill Syrians -- the only way to stop ISIS is to cut off their funding and their weapons... My heart cries for the Syrian people," said protester Jenny Eyeles, 65, using an alternative acronym for IS.
Demonstrators chanted "Don't bomb Syria, no more wars" and carried placards with messages including "Stop the fear, stop the hate, stop the war", echoing another 5,000-strong protest that was held in London on Saturday.
Cameron is keen to join France and the United States in bombing IS targets in Syria, but has held off a vote for months after he was humiliated in 2013 when parliament rejected British military action against Syrian leader Bashar al-Assad.
The Paris attacks have since swayed some lawmakers, including Labour politicians, who have been allowed a free vote on the issue despite opposition to air strikes by anti-war party leader Corbyn.
Underscoring divisions over the move, parliament's foreign affairs committee on Tuesday said Cameron had "not adequately addressed" its concerns on Syria air strikes.
The committee has questioned the legality of the move, its effectiveness in the absence of reliable allies on the ground and its usefulness in the context of finding a diplomatic solution to Syria's civil war.