Insurgents have killed 15 Iraqi soldiers travelling in a convoy south of Baghdad, police and officials say.
Two years on, the protesters' optimism has turned to anger
The attack happened near the town of Latifiya, in a lawless area known as the "triangle of death".
The violence came on the second anniversary of the fall of Baghdad to the US-led coalition.
Tens of thousands of Iraqis joined an anti-US protest in Firdus Square, where Saddam Hussein's statue was toppled on 9 April 2003 as millions watched on TV.
Chanting "No to America" and "No to the occupiers", they pulled down and burned effigies of Saddam Hussein, US President George W Bush and British Prime Minister Tony Blair.
Details of the attack near Latifiya are still sketchy, with conflicting accounts of how the soldiers died.
Police in the nearby town of Mahmudiya told Reuters news agency that gunmen forced the soldiers' truck to stop before shooting and killing them.
However, an Iraqi defence ministry official told AFP news agency that they were blown up by a roadside bomb.
News of the attack came as protesters poured into Firdus Square for a rally called by radical Shia cleric Moqtada Sadr.
They joined a group who had been in the square since Friday night.
Many of the demonstrators had travelled hundreds of miles from Shia cities in southern Iraq to attend the rally. Others came from Baghdad's Sadr City slum, scene of a failed uprising by Mr Sadr's Mehdi Army fighters last year.
TWO YEARS ON
More than 130,000 US troops remain in Iraq
Unofficial estimates of civilian deaths range from at least 15,000 to almost 100,000
Iraqis face fuel shortages and have to buy essential goods at black market prices
Unemployment is estimated at between 25% and 50%
Iraqi security security forces blocked off a number of streets in central Baghdad, while US soldiers kept their distance.
The protest was a peaceful one aimed at urging US troops to leave and demanding quicker trials for Saddam Hussein and his aides, a spokesman for Mr Sadr said.
Sunni clerics from the Association of Muslim Scholars had asked their supporters to join the demonstration.
Two years since Saddam Hussein's statue was torn down, Iraq is now on the verge of having its first elected government in half a century.
However, many Iraqis are frustrated not only at the continuing violence, but also at the lack of jobs and basic services, says the BBC's Caroline Hawley in Baghdad.
One protester from Sadr City, 30-year-old Ali Hussein, told AFP: "The war has been finished for two years. What did we get? There is no electricity, no services, no nothing."
Little has been heard from Mr Sadr in recent months since the uprisings by his followers in many parts of central and southern Iraq last year.
The Mehdi Army militia fought street battles against US forces in Najaf and Sadr City for several months, but in October Mr Sadr urged fighters to hand in their weapons.