The activists filled the tourist hotspot with Syrian and communist flags, plus banners with slogans such as "No Attack on Syria" and "Hands Off Syria" in the rally organised by the left-wing Stop the War Coalition.
Kate Hudson, general secretary of the Campaign for Nuclear Disarmament, told AFP the protesters were calling for the government to stick by Thursday's vote, which saw lawmakers reject Cameron's bid for British involvement in possible US-led military strikes against Syria.
"Every day, dozens if not hundreds of people are killed in Iraq as a direct result of Western intervention. We have to learn the lessons of that and not repeat that in Syria."
Protester Edmond Furse added: "We had misinformation in the Iraq war in 2003 from (US) General (Colin) Powell. We have the same story today but with a twist -- the chemical weapons attack. We don't know if it was from Syrian government forces."
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Meanwhile in north London, volunteers were loading hospital beds into containers destined for Syria at a "Hand in Hand for Syria" collection point.
The charity has collected funds and donations from across Britain and is preparing to ship items and distribute them to civilians affected by the uprising in Syria.
The funds raised have been spent on purchasing medical equipment, while clothing and supplies have been donated by individuals and organisations.