In London, police said some 15,000 protesters joined a rally in the city's Trafalgar Square.
In Spain, some 4,000 protesters marched through central Madrid, in a demonstration organized by leftist political groups.
In Brussels, police said 2,000 people gathered in front of the U.S. Embassy.
Some 50 protesters in the Czech capital Prague handed out antiwar leaflets.
Earlier, hundreds of demonstrators took to the streets in Tokyo, calling on the Japanese government to bring the country's troops home.
In Seoul, some 1,000 activists called for the withdrawal of South Korea's 3,200-member contingent from Iraq.
Yesterday, some 7,000 protesters marched through the streets of Chicago, calling for an immediate U.S. pullout from Iraq.
(compiled from agency reports)
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