In the US, up to 30,000 people marched through New York on Sunday, a day after similar mass protests in Washington and Los Angeles, the US group United for Peace and Justice said. Police did not estimate the numbers.
It said in a statement: "We'll solemnly honour the sacrifice made by more than 3,000 [US] servicemen and women, and we'll contemplate the path ahead of us."
The organisation, which says it has more than three million members, has urged Americans to help stop attempts by the administration of George Bush, the US president, to escalate the war in Iraq.
The action follows protests against the war that have taken place around the world in the past few days.
"Iraq has no future unless this illegal occupation is ended"
Shafiq, Dhaka, Bangladesh
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In Europe, about 5,000 people demonstrated in the Belgian capital, Brussels, on Sunday demanding the withdrawal of US forces from Iraq and the end of the Israeli occupation of Palestinian territory.
They also chanted slogans against US foreign policy towards Lebanon and Iran.
Protests were also held over the weekend in Britain, Australia, Spain and Canada.
'Voice of the people'
Vietnam war veterans joined students and musicians in a march through Manhattan, chanting "Troops out now" and calling for Bush to be impeached.
Tim Robbins, an actor and critic of the Iraq war, helped lead the march. He said: "This is the voice of the people.
"American people want this war to end, so when are we going to start listening to them? The main message is to stop this immoral war."
Jose Vasquez, a conscientious objector US army staff sergeant, who refused to be deployed to Iraq, said he believed the anti-war movement was seeing renewed vigour with the weekend protests.
|Wesley Clark has criticised US strategy in |
Iraq since the invasion four years ago [EPA]
"If people are willing to listen to what the troops have to say, they'll find that the military itself is turning against the war, not unlike what happened during Vietnam," he said.
Wesley Clark, former Nato commander and 2004 US presidential candidate, spoke to Al Jazeera from Doha about the US-led invasion four years ago.
"I think the invasion was a colossal strategic failure... [it] ranks as one of the great strategic blunders in US history.
"It has undercut the notion of American legitimacy and democracy which we nurtured for 200 years.
"It turned out that we were successful in defeating the Iraqi army ... but the emergence of a democratic government didn't happen the way we wanted it.
"What Americans will take [from the war in Iraq] is the importance of using force as a last resort.
"And we didn't do enough work in the region after the war to understand the perspective of all Iraq's neighbours."
On Saturday, thousands of people demanding a US withdrawal from Iraq
marched on the Pentagon, the US defence department's Washington DC headquarters.
Ramsey Clark, the former US attorney general who was among those at the rally, called for Bush to be impeached over his handling of the war.
About 3,200 American soldiers have died since the 2003 US-led invasion of Iraq, according to Reuters news agency.
Iraq Body Count, an organisation estimating the number of civilian deaths since the US-led invasion of Iraq based on news reports, says up to 65,000 Iraqi civilians have died.
An estimate in October by the Lancet, the British medical journal, and the John Hopkins Bloomberg School of Public Health in Baltimore, US, estimated about 655,000 Iraqis, or 2.5 per cent of the population, had been killed as a result of the invasion, although some academics and politicians disputed the figures.
Source: Al Jazeera and agencies