Hundreds of thousands of Iraqi Shias have gathered in the holy city of Najaf for a mass demonstration calling for US-led troops to leave Iraq.
Up to one million people were expected in Najaf after an appeal by Shia cleric Moqtada Sadr, who branded US forces "your arch enemy" in a statement.
He called Iraqis to Najaf to mark four years since US troops entered Baghdad and ended the rule of Saddam Hussein.
Baghdad has been placed under curfew for the duration of the anniversary.
A 24-hour ban on movement by all vehicles, for fear of car bomb attacks, began in the city at 0500 (0100 GMT) on Monday, where four years ago a giant statue of Saddam Hussein was torn down, symbolising the fall of his regime.
Followers of Moqtada Sadr play a key role in Iraq, with the Mehdi Army said to be heavily involved in the sectarian violence of the past year.
The BBC's Jim Muir, in Baghdad, says the Americans regard the cleric and his militia as the biggest danger to Iraq today.
However, the militia is reported to have stood down in response to a nearly eight-week-old US "surge", or security drive in Baghdad.
The cleric did not appear personally, but called for the mass protest in a statement issued on Sunday.
"In order to end the occupation, you will go out and demonstrate," he said.
He asked Iraqis not to "walk alongside the occupiers, because they are your arch enemy" and to turn all their efforts on US forces.
But he warned followers against violence, urging the Mehdi Army and Iraqi security forces "to be to be patient and to unite your efforts against the enemy and not against the sons of Iraq".
One members of Mr Sadr's organisation, Salah al-Obaydi, called the rally a "call for liberation".
"We're hoping that by next year's anniversary, we will be an independent and liberated Iraq with full sovereignty," he told the Associated Press.
If the American people and the Iraqi people want the US out of Iraq, why are they still there?