With less than a day to go before MPs and peers were due to debate the government's proposals, a demonstration took place opposite the entrance to Downing Street.
Several hundred people, ranging from teenagers to the elderly, held placards bearing slogans such as "Hands off Syria" and "Cut war, not welfare".
Ann-Kristine WestwoodAnn-Kristine Westwood says the UK government and its allies have acted like terrorists
Ann-Kristine Westwood, a grandmother from Nuneaton, Warwickshire, told the BBC: "I can't see what we've achieved in 10 years in Iraq or Afghanistan. And I'm convinced we are being sold another pup.
"We can't afford schools, houses, teachers, nurses, but we buy as many bombs as we want. The people in this country can't afford another war."
She added: "The mothers in Syria are going to be terrified. We are now the terrorists."