The US military said on Friday it was sending some 2,600 soldiers to Iraq earlier than planned, raising the number of extra US troops being deployed, in a new effort to stabilise the country, to nearly 30,000.
Around 500 protesters marched through Sydney on Saturday to mark the fourth anniversary of the invasion of Iraq.
Similar rallies were held in cities around Australia.
The demonstrators called for the withdrawal of troops from not just Iraq but Afghanistan as well, and for the government of John Howard, Australia's prime minister, to bring Australian terrorism suspect David Hicks home from US detention at Guantanamo Bay in Cuba.
Kerry Nettle, a Greens member of parliament, said she had campaigned against Australian participation when it was first mooted.
Nettle said: "Four years on, how things have changed. The issue of Iraq is the disaster we all said it would be. The predictions have not only come true, it's worse."
It's been 40 years since protesters mobilised a massive march on the Pentagon in Washington against the Vietnam war - an image that organisers of Saturday's massive march against the Iraq war are hoping to match.
"Peace in Iraq will happen when citizens realise that they can express their ideas and views more effectively... than with violence"
JBernar5, Toledo, USA
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Bill Hackwell, a spokesman for the Act Now to Stop War and End Racism (Answer) coalition, which is organising the march, said: "We're feeling a shift in the general population of the country who are now opposed to the war and are now thinking about doing something about it, not only about voting but becoming active in the anti-war struggle."
The 1967 Pentagon march, which looms large in the nation's collective memory as the beginning of the end of the war in Vietnam, drew an estimated 50,000 to 100,00 marchers to Washington - a number that will likely be used as a measure for the success of Saturday's march.
Late on Friday, about 100 peace activists, including members of the clergy, were arrested as they held a vigil on a sidewalk in front of the White House, ignoring police orders to disperse.
Kate Manzanares, a 29-year-old doctor from Virginia, who watched the arrests along with about 1,000 other protesters, said: "Hopefully, I think it really speaks truth to power."
Christian prayer service
Earlier in the day, the group that organised the vigil, Christian Peace Witness for Iraq, held a service at Washington's National Cathedral that was attended by 3,500-4,000 people, organisers said.
The war has grown increasingly unpopular, with recent polls showing that a majority of Americans now say the invasion was a mistake and want the US government to set a timetable for the withdrawal of US troops from Iraq.
Hackwell said more Americans are demanding change because too much money is being spent on the war rather than social needs like health care, education and employment.
He said: "It is not an exaggeration to make this connection about the funding that gets cut for needed social programmes and the money going to the Pentagon."
Other anti-war events are planned in the next few days across the country.
In Los Angeles, thousands of protesters are expected to take the streets of Hollywood on Saturday and another Answer demonstration is expected in San Francisco. The anti-war coalition United for Peace and Justice said it was expecting thousands of people to turn up at a protest in New York on Sunday.
Activists want the Democratic-led Congress to push hard for the withdrawal of US troops from Iraq.
Bulking up about 140,000 US troops already fighting in Iraq, the combat aviation brigade from the US army's third infantry division will deploy in early May, some 45 days sooner than previously envisaged, the US military said on Friday.
|Demonstrators held a huge protest rally|
in central Nicosia in Cyprus on Saturday [EPA]
The brigade is the third element to be announced in a package of support units being deployed to assist 21,500 extra combat troops ordered to Iraq under a plan unveiled by George Bush, the US president, in January.
Robert Gates, the US defence secretary, said: "The aviation brigade, which is really principally rotary helicopter support for the troops, is the final piece."
Sectarian violence in Iraq has continuously thwarted American efforts to bring the four-year-old war to a close.