Fiery Shi'ite cleric Moqtada al-Sadr's call for a big anti-U.S. protest on Monday was answered by thousands of Iraqis who flocked to the southern holy city of Najaf.

Sadr, who blames the U.S.-led invasion for Iraq's unrelenting violence, issued a statement on Sunday urging Iraqis to protest on the fourth anniversary of the day U.S. forces swept into central Baghdad.

"In order to end the occupation, you will go out and demonstrate," said Sadr, who had been keeping a low profile in the last few weeks.

The U.S. military says Sadr, who is popular among Iraq's urban Shi'ite Muslim poor, is in neighboring Iran. His aides say the cleric is in Iraq and have denied suggestions he fled to Iran to escape a security crackdown in Baghdad.

Thousands of Shi'ites traveled in buses or cars to Najaf in response to Sadr's call. The Baghdad-Najaf road was packed with hundreds of vehicles crammed with passengers waving Iraqi flags and chanting religious and anti-U.S. slogans.

"No, no, no to America ... Moqtada, yes, yes, yes," they chanted as they converged on the holy city.

Sadr called on his Mehdi Army militia and Iraqi security forces to stop fighting in the volatile city of Diwaniya so as not to play into the hands of U.S. forces, who he said had stirred up civil strife in Iraq.

Iraqi and U.S. forces have clashed with militiamen in Diwaniya since launching an operation on Friday to wrest control of the southern city from the Mehdi Army. The Pentagon says Sadr's militia is the greatest threat to peace in Iraq.

VEHICLE BAN

Brigadier Qassim Moussawi, spokesman for the U.S.-Iraqi security crackdown in Baghdad, said a 24-hour vehicle ban would be in force in the capital on Monday.

"There will be protests marking the fourth anniversary. We don't want to give the terrorists a chance to use this opportunity," Moussawi said.

What largely began as a Sunni Arab insurgency against U.S. and Iraqi forces after the 2003 invasion of Iraq has since widened into a sectarian conflict between the country's Shi'ite majority and Sunnis, once dominant under Saddam Hussein.

Tens of thousands of Iraqis have been killed in the past year alone. More than 3,270 U.S. soldiers have been killed since the U.S.-led invasion.

A car bomb killed 17 people and wounded two dozen in the town of Mahmudiya south of Baghdad on Sunday, officials said.

A suicide car bomber killed seven people in Baghdad.

Four U.S. soldiers were killed in attacks south of Baghdad on Sunday while another two died from wounds suffered in operations north of the capital, the U.S. military said.

The toll made it a deadly weekend for U.S. troops after four soldiers were killed in an explosion near their vehicle in Diyala province north of Baghdad on Saturday.