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By Khaled Farhan

NAJAF, Iraq, April 9 (Reuters) - Baghdad was under curfew on Monday, the fourth anniversary of the fall of the capital to U.S. forces, as Iraqis gathered in the city of Najaf for a big anti-U.S. protest called by fiery cleric Moqtada al-Sadr.

"No, no, to the occupation, no, no to America," thousands of marching Iraqis, mainly men and young boys waving Iraqi flags, chanted as they marched through the southern Shi'ite holy city.

Iraq imposed a 24-hour vehicle ban in Baghdad from 5 a.m. (0100 GMT) to prevent any attacks on the anniversary. Car bombs still plague the capital, despite a new security crackdown by tens of thousands of U.S. and Iraqi troops that is seen as a last attempt to avert sectarian civil war.

Sadr, who blames the U.S.-led invasion for Iraq's unrelenting violence, issued a statement on Sunday urging Iraqis to protest against the presence of U.S. troops in Iraq.

Protesters in Najaf burnt the American flag and spray painted the slogans "May America fall" and "Bush is a dog" on the ground. Thousands were marching from nearby Kufa, while others clogged roads as they came by car and bus from Baghdad and Shi'ite cities in the south.

The powerful young cleric led two uprisings against U.S. forces in 2004 but has since become a major political player. His movement holds a quarter of the seats in the ruling Shi'ite Alliance of Prime Minister Nuri al-Maliki.

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

U.S. President George W. Bush has insisted U.S. troops will not leave until Iraqis can take over security and has repeatedly rejected setting a timetable for withdrawal.

The U.S. military says Sadr, who leads the Mehdi Army militia which is blamed for fuelling sectarian strife with minority Sunni Muslims, is in neighbouring Iran. His aides say the cleric is in Iraq and have denied suggestions he fled to escape the security crackdown.

His Mehdi army fought gunbattles with U.S. and Iraqi forces in the southern city of Diwaniya at the weekend after the troops swept into neighbourhoods to hunt for militiamen.

ANNIVERSARY

Four years ago to the day, the world watched as Iraqis, helped by U.S. soldiers, toppled Saddam's 20-foot statue in Baghdad's central Firdous Square. A crowd swarmed over what was left of the statue and danced for joy.

Saddam had vowed to defeat the U.S.-led invasion launched on March 20, 2003, but his forces offered little resistance as U.S. forces thrust deep into the heart of the Iraqi capital.

By then the war had cost 96 American dead, 30 British dead and unknown thousands of Iraqi military and civilian casualties.

Four years on those tolls have soared to more than 3,270 U.S. soldiers killed, 140 British soldiers, 124 from other nations, and tens of thousands of Iraqis.

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 brought to 10 the number of soldiers killed at the weekend and 35 in the first eight days of April.

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

"This day is a good one because it is the anniversary of the fall of Saddam, but the security is poor. There's no water, no electricity. The situation is poor everywhere," said one Baghdad resident, Hatem Karim.

U.S. military commanders counter that the capital now gets up to nine hours of electricity a day and that the security crackdown has succeeded in reducing the daily killings by sectarian hit squads and needs more time to show results.

Bush is sending 30,000 more troops to Iraq, mainly for the Baghdad operation. (Additional reporting by Yara Bayoumy and Mussab Al-Khairalla in Baghdad)

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