Demonstrations were held in New York, Washington, San Francisco, Los Angeles and other cities across the US.
They come at a time when anti-war sentiment in the US is at an all-time high and the popularity of President George W. Bush has plummeted.
Demonstrations were also held across Europe.
The biggest was in Rome, where tens of thousands marched through the city.
The ruling centre-right government of Prime Minister Silvio Berlusconi, a staunch ally of US President George W. Bush, sent nearly 3,000 Italian soldiers to Iraq, despite the objections of a majority of Italians.
At least 15,000 marched through London, demanding the withdrawal of 8,000 British troops from Iraq.
Thousands also turned out in Spain, Denmark, Greece and Turkey.