Labour MP Emma Dent Coad has handed a letter in to Downing Street urging Theresa May not to take military action in Syria.
The handover of the letter, co-ordinated by the Stop the War Coalition, was marked with a protest in Whitehall attended by a large numbers of activists.
The letter has been signed by MPs, trade unionists, celebrities and academics.
Speaking after the letter was handed in, Ms Dent Coad said the current case for intervention was based on supposition and that military action should be put to a vote in Parliament.
"Syria has been bombed to bits already. Are we really going to go in and finish the job for Assad? I think that's really frightening," she said.
"There's no precision bombing in Syria. It's so overlapping. There's no good guys and bad guys - everybody is mixed up together.
"If we bomb airbases, we could be bombing Russians. We are at the brink of something really frightening.
"We have to have proper evidence, we have to know what we're doing. No more war without full evidence and full UN backing."