From Sydney to Tokyo, London to New York and San Francisco and across hundreds of other cities and towns, millions of protesters poured into the streets to condemn what they believed was an "unjust, illegitimate" war.
Journalists estimated that at least a million people streamed through Rome, in probably the biggest single protest during the day.
Climbing Big Ben
In London, two anti-war demonstrators climbed up the landmark Big Ben clock tower at the House of Parliament, unfurling a banner reading "Time for Truth".
The troubling truth one year on has been that the weapons of mass destruction - the pretext on which the US and UK led the invasion - are still nowhere to be found.
Two demonstrators climbed
Big Ben to vent their outrage
About 25,000 demonstrators gathered in central London, many carrying 'Wanted" posters bearing images of US President George Bush and UK Prime Minister Tony Blair.
In New York, tens of thousands created a sea of signs in midtown Manhattan. Among the signs spotted in the crowd were, "Money for Jobs and Education and not for War and Occupation".
Anti-war activists gathered at a park in the small central Texas town of Crawford, but out of sight of Bush's ranch there.
"I hate George Bush and everything he stands for and this war of vanity," said Don Marshburn, a Navy veteran.
"I am sick of bombs. It didn't do anything over there and it didn't do anything over here," he said.
New York's crowd was the largest in the US, with organisers estimating up to 100,000 protesters.
"Hey Hey, Ho Ho, George Bush has got to go," marchers chanted.
Similar sentiments echoed through cities and towns on other continents.
In Tokyo, thousands of people marched through the centre of the city on Saturday to call for peace in Iraq.
Despite pouring rain, people gathered at Tokyo's Hibiya Park for an anti-war rally before the march, demanding the United States and Japan should immediately withdraw from Iraq.
"Hey Hey, Ho Ho, George Bush has got to go"
Anti-war protesters in New York
Many carried banners and placards with slogans including "US, Get Out of Iraq Now!", "Stop the Occupation!" and "No to Japan's Dispatch of Ground Troops to Iraq."
Organisers claimed 30,000 people had turned out.
A middle-aged man held a large poster that showed Bush, Blair and Japanese Prime Minister Junichiro Koizumi under the headline: "The Worst Terrorists."
Another man in his early 20s carried a card that said: "Mr Bush, where are the weapons of mass destruction? You should get rid of your weapons of mass destruction in the US."
In Sydney, Australia's biggest city, more than 3000 chanting: "End the occupation, troops out" marched through the city centre, carrying a caged effigy of the head of Australian Prime Minister John Howard, a staunch supporter of the war.
Filmmaker and writer John Pilger and actress Judy Davis addressed the rally. In Melbourne the father of Australian David Hicks, being held as a "terror suspect" in a US military prison in Guantanamo Bay, Cuba, called for justice for his son.
"David, if he has done anything wrong, should have been charged or released two years ago," Terry Hicks told the demonstrators.
Australia, a close ally of the US, sent 2000 troops to Iraq and also dispatched special forces troops to Afghanistan. About 850 of its troops remain in the Gulf.
Tens of thousands of people staged a mass protest in Rome on Saturday against the US-led occupation of Iraq which had the support of the Italian government.
The protesters - numbering 300,000 according to organisers - filed through the city centre, marching under a rainbow-coloured flag and a giant banner reading "Together for Peace".
A Filipino protester is sprayed
with a water cannon
The rally was called by the left-wing opposition, trade unions, non-governmental organisations and local government figures in protest at the pro-US foreign policy of Prime Minister Silvio Berlusconi's centre-right government.
"We are against war and against terrorism," cried demonstrators, in response to the charge by members of the Italian government that the rally played into the hands of Islamist extremists.
Also on Saturday, Philippine riot police used water cannons to disperse a group of about 100 demonstrators who tried to march on the US embassy to stage a war protest.
Members of the left-wing New Nationalist Alliance (Bayan),
waving signs saying "Stop the War," had earlier staged a protest a short distance from the embassy condemning the US government and Philippine President Gloria Arroyo for supporting the US-led attack and the coalition forces occupying Iraq.
However, they ignored police warnings they would not be allowed to get near the embassy and tried to march on the area, prompting police to use water cannons to break them up.
Other leftist and nationalist groups staged peaceful protests
against the Iraq invasion in other parts of Manila. Vice-President Teofisto Guingona, who split with Arroyo over her close security ties with the United States, was due to take part in one of the rallies.