NAJAF, Iraq - Iraq's most powerful Shiite cleric returned to the country from Britain today and his aides called for a nationwide march to Najaf to end nearly three weeks of fierce fighting between U.S. forces and Shiite militants in this holy city.

The announcement came as heavy fighting persisted in Najaf's old city. U.S. warplanes fired on the neighborhood, helicopters flew overhead and heavy gunfire was heard in the streets, witnesses said.

Iraqi police sealed the area, preventing cars from entering, and Najaf's police chief, Maj. Gen. Ghalib al-Jazaari, said radical Shiite cleric Muqtada al-Sadr's Mahdi Army militia was on its last legs.

"The Mahdi Army is finished," he said. "Its hours are numbered."

Militants were still fighting in the streets of the Old City, witnesses said, though the relentless American attacks in Najaf appeared to be weakening them.

Police today arrested several al-Sadr aides with valuables in their possession from the sacred Imam Ali Shrine, which they control, al-Jazaari said. One of al-Sadr's top lieutenants, Sheik Ali Smeisim, was among those arrested, police officials said on condition of anonymity.

Grand Ayatollah Ali Husseini al-Sistani, 73, the nation's top Shiite cleric, crossed into southern Iraq from Kuwait about midday in a caravan of sport utility vehicles accompanied by Iraqi police and national guardsmen, according to an Associated Press reporter with the convoy. The convoy stopped in the southern city of Basra.

Al-Sistani had been in London for medical treatment since Aug. 6, one day after clashes erupted in Najaf. The cleric wields enormous influence among Shiite Iraqis and his return could play a crucial role in stabilizing the crisis.

Al-Sistani would head to Najaf on Thursday "to stop the bloodshed," said Al-Sayyid Murtadha Al-Kashmiri, an al-Sistani representative in London. "Those believers who wish to join him, let them join."

The police chief cautioned Iraqis not to come to Najaf, saying they should await instructions from al-Sistani, "because their enemies could cause them a disaster and they could put their lives in danger."

In separate violence west of Baghdad, U.S. warplanes and tanks bombed the volatile city of Fallujah for more than two hours, killing at least four people and wounding four others, hospital officials and residents said.

Sunni insurgents based in the city are believed to be responsible for months of kidnappings, bombings and shooting attacks against coalition troops, Iraqi forces and civilians.

Marine spokesman Lt. Col. Thomas V. Johnson said today that several insurgent firing positions had been struck.