Demonstrations were held globally, but in the US, a country with a population of 298 million, the events drew only about 1000 people in major cities.
Yet anti-war sentiment in the US is at an all-time high and the popularity of George Bush, president, the architect of the war, has plummeted.
The low US turnout was mirrored in anti-war protests in most other countries.
Demonstrations in the US were organised in New York, Washington, Los Angeles, San Francisco and a number of smaller cities by several groups, including the ANSWER (Act Now to Stop War and End Racism) coalition.
At a rally of 1000 protesters near Times Square in New York, speaker after speaker denounced the Bush administration and the continuing US troop presence in Iraq.
The demonstration, organised by the group Troops Out Now, called for an immediate, complete and unconditional US military withdrawal.
Organiser Dustin Langley said: "Public opinion is now overwhelmingly on our side as it becomes clearer every day that this occupation itself is the source of the violence in Iraq.
Protesters march down 42nd
Street in New York
"But politicians of neither party are going to end the war, so we have to get back on the streets."
In Washington, about a thousand protesters gathered outside the residence of Dick Cheney, the vice-president. "This racist war has to go," they chanted, some carrying signs reading "Bush step down", "Impeachment now" and "Hands off Iran".
On the West Coast, celebrities joined protesters at the legendary intersection of Hollywood Boulevard and Vine Street.
Kirk Albanese, commander of the Los Angeles Police Deparment, said: "Twelve hundred protesters were marching in Hollywood today."
Among them were Paul Haggis, the Canadian director of Crash, this year's Oscar winner for best picture, and actors Martin Sheen, Laura Dern and Maria Bello.
Also participating were Vietnam War veteran Ron Kovic, the author of the book Born on the Fourth of July, which later became an academy-award winning film, and Dolores Huerta, co-founder of the United Farm Workers of America.
Marcial Guerra, of ANSWER, which organised the Los Angeles protest, said: "We want to put an end to this insane war and to spend money where it is needed, for example on the people hurt by Hurricane Katrina."
Bush, in his weekly radio address, insisted that invading Iraq was "the right decision," and vowed to overcome violence there that has killed 2300 US soldiers and countless Iraqi civilians.
"We will finish the mission," said Bush. "By defeating the terrorists in Iraq, we will bring greater security to our own country."
Bush did not mention the failure to find Iraq's alleged arsenal of nuclear, biological and chemical weapons that was the core of his public case for war.