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.
Former Labour cabinet minister Peter Hain told the Guardian he felt intervening in Syria would be "very dangerous".
Mr Hain, who is poised to vote against the motion when Parliament meets tomorrow if he returns from a foreign trip in time, said: "The Prime Minister is asking the nation to back him on a dangerous strike when nobody knows what the consequences will be. I think that's very, very dangerous politics."
Former Welsh Secretary Peter Hain said his colleague Andy Burnham "speaks for all Labour supporters" after he criticised the direction of the Labour Party.
Mr Hain wrote on Twitter:
Mr Burnham's comments have been interpreted as an attack on Labour leader Ed Miliband - a claim the shadow health secretary denies.
Former Cabinet minister Peter Hain has expressed anger after hackers targeted his website.
The Labour MP's site was taken over by the Anonymous group, who left messages criticising the politician's record and supporting Wikileaks founder Julian Assange.
In a statement, the Neath MP said: "My website was hacked by Anonymous this morning and we are working to restore it as soon as possible.
"After the targeting of several high-profile websites in recent months, this latest incident is yet more evidence that we all need to wake up to the growing cyber security threat in Britain."