The protests, the first major demonstrations since the fall of Saddam Hussein in April, were held as the United States tried to gain international help in rebuilding Iraq, where American troops regularly come under attack. The demonstrations were organized in each country by local activist groups that have informal contacts with each other.
London's was the biggest protest, drawing 20,000 people. Demonstrators turned out in a dozen other countries, including South Korea and Egypt.
"No more war. No more lies" proclaimed a banner pinned to the pedestal of Nelson's Column in London's Trafalgar Square where demonstrators rallied after a march through the city. People of all ages, from gray-haired couples to toddlers in strollers, joined the orderly stream of protesters marching from Hyde Park.
Some young marchers chanted, "George Bush, Uncle Sam, Iraq will be your Vietnam!"
CBS News Corresponent Mark Phillips reports from London that the rally's organizers were hoping for 100 thousand demonstrators. And though police put the number at 20 thousand, "Either way this was never going to be as big or as broadly based as the massive demonstration that proceeded the war."
Still, says Phillips, "A million or so people demonstrating here before the war didn't keep it from happening. And this rally won't end the occupation. But at a time of growing skepticism with the government here as to why it went to war in the first place, this will serve to keep the argument alive."
Demonstrators, including those in London, also added the Palestinian cause to their campaign.
Some 3,000 people marched in Paris, where a wide banner read, "American Imperialism: Take your bloody hands off the Middle East." Others held posters that read "Wanted: George W. Bush - War Criminal."
Outside the U.S. Embassy in Athens, demonstrators hurled bottles and yoghurt at riot police during a rally to protest the occupation of Iraq and the Palestinian territories. About 3,000 protesters, chanting "Occupiers Out" and "Freedom for Palestine," joined the rally.
Protests were also staged in other parts of Greece and on island of Crete, outside an American naval base at Souda Bay. The base supports the U.S. 6th Fleet and spy planes.
In Seoul, thousands of activists protested a U.S. request to send South Korean troops to Iraq. Protesters chanted "No war!" and carried banners saying "End the occupation in Iraq" and "Oppose a plan to dispatch S. Korean combat troops to Iraq"
Some 4,000 protesters in the Turkish capital, Ankara, shouted slogans and unfurled banners to support the Palestinian cause and demand an end to the U.S.-led occupation of Iraq. Hundreds more gathered at a similar rally in Istanbul and burned American and Israeli flags.
In downtown Cairo, about 50 political activists and journalists staged a peaceful protest against the U.S.-led occupation of Iraq and Israeli attacks on Palestinians.
In Warsaw, 100 young people protested the Polish military presence in Iraq, marching with banners saying "Down with the global U.S terrorism" and "We don't want to occupy with Bush."
An estimated 1,200 demonstrated in Brussels, while about 400 people marched through downtown Berlin. In Stockholm, police said about 250 people staged a demonstration.
Opposition to the war has always been strong in Britain. Several large peace protests were held during the war, though none matched the vast rally Feb. 15, before the conflict began, when between 750,000 and 2 million people marched through central London.
Now, questions about Prime Minister Tony Blair's tactics in trying to win public support before invading Iraq have left his government struggling through its worst crisis. The ruling Labor Party is still well ahead of the opposition in opinion polls, but the public's faith in the government and in Blair personally has eroded.
A new poll taken Sept. 11-16 and published Saturday in The Financial Times found 50 percent of those questioned said Blair should step aside and let someone else in the party lead the country. The newspaper did not give the sample size or margin of error.
The London protest Saturday was timed for the eve of the governing party's annual conference for "maximum political impact," said Andrew Burgin, spokesman for Stop the War Coalition, one of the rally's organizers.
Campaign for Nuclear Disarmament, another organizer of Saturday's march, said a big demonstration would send a strong message to the government that the public did not condone what it called "lies" used to justify the war.
Opponents of Blair reject the justification he used for war - the threat of Iraqi biological and chemical weapons. No such weapons have yet been found.
"I don't believe the war with Iraq was right and the proof is we haven't found any weapons of mass destruction," said London protester Emma Loebid, 20. "I think they should hand Iraq back to the Iraqis and get the troops out."
Twenty-year-old Liban Kahiye, also in London, said, "I don't believe British and American troops should still be in Iraq. Everyday you hear stories of innocent people being killed - that's not justice."