Speakers at the event, organised by the Stop the War coalition, hailed Parliament’s vote on Thursday against British military strikes as a victory for peace campaigners.
Lindsey German, convener of the Stop The War coalition, said: “Never let them say demonstrations don’t work – our demonstration has worked.”
Former Labour MP Tony Benn said: “Today is a victory of British public opinion over those who want war and the vote in Parliament last week was a result of all the demonstrations, like the one we had today, against war and in favour of peace.
“It is an indication that the British government, which is a very right wing government, has had to listen to public opinion.
“Chemical weapons are terrible weapons but when you think of all the thousands of people that would that have been killed by British and American troops in Afghanistan and Iraq you realise that it isn’t true that another war would solve the problem.”
Conservative rebels joined with Labour on Thursday to defeat a government motion backing the use of force “if necessary”.
The motion was in response to an alleged chemical weapons attack by the Syrian government against rebels in the suburbs of Damascus earlier this month.
Speakers addressed the crowd from below Nelson’s Column. About 1,000 campaigners carried banners with slogans such as No Attack on Syria and Hands Off Syria as well as Syrian flags.
Women with Syrian flags painted on their faces chanted “USA shame on you”. There were also several trade union and Communist flags with the hammer and sickle on display.
Stop the War coalition said that 5,000 people had turned out to support the rally.