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The protesters were responding to an appeal by cleric Moqtada Sadr, who branded US forces "your arch enemy" in a statement.

The demonstration marks 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.

US praise

The protest in Najaf, 160km (100 miles) south of Baghdad, broke up after about three hours.

The BBC's Jim Muir in Baghdad says there were no reports of violence as protesters waved flags, sang and chanted slogans calling for an end to the occupation.

There was no sign of Moqtada Sadr, who has not been seen in public since US and Iraqi army forces began a new security drive in and around Baghdad nearly eight weeks ago.

The US believes he is in Iran. Our correspondent says the Americans regard the cleric and his militia, the Mehdi Army, as the biggest danger to Iraq today.

The militia is said to be heavily involved in the sectarian violence of the past year, although it was reported to have stood down in response to the security "surge", which involves an extra 30,000 US troops.

The US military praised the peaceful nature of the protest.

Spokesman Col Steven Boylan said: "This is the right to assemble, the right to free speech - they didn't have that under the former regime."

In a statement issued on Sunday, the cleric 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".

Japan loans

Thousands of Shias had headed to Najaf in tightly-packed buses and cars.

Some demonstrators burned US flags and shouted slogans: "No, no, no to America... Moqtada, yes, yes, yes.".

One member 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.

Clashes continued on Monday between coalition forces and followers loyal to the cleric in the town of Diwaniya.

US and Iraqi forces began an operation to root out militiamen there on Friday.

Moqtada Sadr's supporters hold a crucial block of seats in Iraq's parliament, giving them an influential voice in Prime Minister Nouri Maliki's government.

Mr Maliki is in Tokyo where he met Prime Minister Shinzo Abe.

They signed previously agreed loans worth 103bn yen ($86bn; £44bn) for four economic projects.

Mr Maliki thanked Tokyo for its support and said: "Iraq has escaped the constraints of the past and is engaged in new challenges. The country is one and the people are one."