Insurgents have killed 15 Iraqi soldiers travelling in a convoy south of Baghdad, police and officials say. 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.
News of the attack came as protesters poured into Firdus Square, carrying banners and chanting anti-US slogans, for a protest called by radical Shia cleric Moqtada Sadr.
They joined a group who had been in the square since Friday night.
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%
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.