Crowds attacked an effigy of President Bush
Thousands of people have protested in Baghdad against a proposed deal to allow US troops to remain in Iraq once their UN mandate expires.
The demonstration was called by the Shia cleric, Moqtada Sadr, who has strongly opposed any deal with the US.
Security was tight around the area, but the organisers insist the protest in the capital will remain peaceful.
The Status of Forces Agreement has been passed by the Iraqi cabinet, but still needs parliamentary approval.
A failure to ratify the agreement or renew the UN mandate might mean US military operations would have to be suspended. The UN mandate for the US-led coalition expires at the end of December.
The Iraqi parliament is expected to vote next week on the proposals, which have also been criticised by other groups.
The demonstration took place in Firdous Square, the scene of the toppling of a statue of Iraq's former leader, Saddam Hussein, five years ago.
Hundreds of Iraqi police searched everyone entering the square and US surveillance drones circled overhead.
The BBC's Andrew North, who was at the protest, says people waved Iraqi flags, Shia banners and chanted anti-American slogans, and an effigy of US President George W Bush was attacked by the crowd.
Joint prayers were held with many Sunni Muslims at the demonstration, our correspondent said.
Moqtada Sadr did not attend the demonstration himself but a sermon of his was read out by a representative, Sheik Abdul-Hadi al-Mohammadawi.
"Let the government know that America is and will not be of any use to us because it is the enemy of Islam," he said.
"The government must know that it is the people who help it in the good and the bad times. If it throws the occupier out, all the Iraqi people will stand by it."
A senior aide to Moqtada Sadr said people were "coming out to prove the security pact is worthless".
"Today is the day of Iraqi unity among Arabs, Kurds, all communities of Iraq, to reject the security pact," said Hazim al-Araji.
Mr Araji said the protest would "express rejection of the pact and the occupation," and that the crowds would remain "100%" peaceful.
Under the forces agreement, US troops will withdraw from the streets of Iraqi towns and cities next year, with all 150,000 having left Iraq by the end of 2011.
The UN mandate for the US-led coalition expires at the end of 2008
Iraqi Prime Minister Nouri Maliki has said the deal is "a solid prelude to the restoration of Iraq's full sovereignty in three years' time" and has insisted that it has no hidden clauses.
Our correspondent says there is little chance that Moqtada Sadr's supporters can stop the agreement being passed.
The event is being seen by many as showing the continuing influence of the cleric, and that he is securing his position for when the Americans do eventually leave, our correspondent adds.
About 144,000 of the 152,000 foreign troops deployed in Iraq are US military personnel.