The protestors marched through some of the busiest parts of the city sometimes blocking traffic.
Against any action
They're against military action which George Bush and Tony Blair have proposed.
One marcher, 14-year-old Hussein Mohammed, was born in Iraq but came to London with his family seven years ago.
He said he doesn't want innocent people killed in an attack but he doesn't think Saddam Hussein should be President anymore.
"I'm against Saddam because he doesn't have a heart for any people," he said.