A message of support has been read to the crowd in Trafalgar Square from former Labour cabinet minister Peter Hain. It said:
"Politics, not warmongering, must be the way forward."
Peter Hain did not attend the vote in Parliament on Thursday because he was abroad.
David Cameron has backed Barack Obama after the president announced he is seeking congressional support to launch military action in Syria.
David Cameron accepted the public did not want intervention in Syria. But he might have got a different result if his ministers voted.