Thousands of protesters gathered in central London in an effort to stop Britain joining air strikes on Islamic State (IS) targets in Syria, a day before a parliamentary vote on the move.
A crowd of around 4,000 marched on Tuesday from the Houses of Parliament in Westminster to the headquarters of the ruling Conservative party and main opposition Labour party nearby.
"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 you make the war even worse."
Parliament looks set to vote in favour of joining the bombing campaign against Islamic State jihadists in Syria on Wednesday, paving the way for sorties by British jets to start within days.
Prime Minister David Cameron insists military action is needed to prevent attacks like the ones that killed 130 people in Paris last month, but many experts, MPs and members of the public remain sceptical.