British police said on Saturday more than a half-million protesters marched in the London neighborhood of Hyde Park. The rally, which included top politicians and celebrities, was thought to be the British capital's largest anti-war protest in many years.
Rallies across France drew at least 400-thousand people, demanding a peaceful resolution to the dispute with Iraq.
In New York, protesters gathered near the United Nations headquarters to hear a scheduled peace message from South African Archbishop Desmond Tutu. Organizers say they expect 100-thousand people.
Protests against war in Iraq also drew huge crowds in Rome and Berlin, and in many Middle Eastern countries.
Peace rallies began Saturday in Auckland, New Zealand and several Australian cities. Protesters across Asia took to the streets in Tokyo, Manila and Bangkok.
Organizers say they expect about 600 rallies and marches worldwide in what looks to be some of the biggest anti-war protests in more than 30 years.