Tens of thousands of Iraqis as well as some 3,200 US soldiers have been killed since the war began on March 20, 2003.

The largest demonstration was in Madrid where 400,000 protestors flooded the streets of the Spanish capital, according to organizers. Protesters marched to a memorial for the 191 people killed in the March 11, 2004 train bombings.

Protesters also held banners reading "Troops out of Afghanistan" and "Zapatero you're not alone," in support of Prime Minister Jose Luis Rodriguez Zapatero's earlier decision to pull Spanish troops out of Iraq.

Similar demonstrations also took place in the Spanish cities of Valencia, Sevilla and Barcelona.

In the Turkish city of Istanbul, about 6,000 protesters took to the streets, waving anti-war flags and signs.

Demonstrations were also held in Greece, Hungary and Cyprus. In Hungary, some 2,000 Hungarians carrying torches marched in and outside the capital Budapest.

Across the Atlantic Ocean, thousands of protestors on Saturday gathered in Washington, D.C., the US capital, where speakers at a rally in front of the Lincoln Memorial denounced the war policy of the Bush administration, and many demanded the immediate withdrawal of US troops from Iraq.

Carrying banners of "US Out of Iraq Now" and "Stop Iraq War, No Iran War, Impeach," the protestors marched from the National Mall, across the Memorial Bridge, and to another rally near the Pentagon.

There were no official estimates of the number of participants, but some news reports put the number at about 10,000 to 20,000, which was less than a third of the massive showing organizers called for. They had hoped for an outpouring similar to the 1967 march on the Pentagon calling for the end of the Vietnam War.

Activists in Iraq and South Korea also held demonstrations against the war.

US President George W. Bush did not respond to any of the anti-war criticism, as he was spending his weekend at the presidential retreat in Camp David, Maryland.

But referring to the protests, White House Spokesman Blair Jones said: "Our Constitution guarantees the right to peacefully express one's views. The men and women in our military are fighting to bring the people of Iraq the same rights and freedoms."

The worldwide anti-war protests came when insurgents in western Iraq set off three chlorine gas car bombs on Friday, according to US forces, sparking fears of a new campaign using unconventional weapons in Iraq.

Recent polls in the United States has shown increasing unpopularity of the war among Americans, with a majority of the respondents now say the invasion was a mistake and want the United States to set a timetable for the withdrawal of U.S. troops from Iraq.

The war has caused a wave of political changes across the Atlantic.

In 2004, Spain's former Prime Minister Jose Maria Aznar was ousted from power only two days after the March 11 Madrid train bombings, which the bombers said it was a revenge on Aznar's move to send Spanish troops to Iraq. Similarly in America's November elections, the Democrats secured their power in Congress marked by voter anger at the war.

But Democrats have so far failed to pass legislation that would compel Bush to change course in Iraq, although a measure calling for a pullout by September 2008 passed a key panel in the House of Representatives on Thursday.