Iraq's most senior Shiite Muslim cleric made a surprise return to the country Wednesday following medical treatment in London and urged "all believers" to join him here to bring an end to three weeks of fighting between U.S. forces and a Shiite militia force loyal to rebel cleric Moqtada Sadr.

Grand Ayatollah Ali Sistani entered Iraq from Kuwait in a convoy guarded by Iraqi police and settled in the southern city of Basra for the night with plans to drive the 230 miles to Najaf on Thursday. Once he reaches this holy city, his aides said, he will lead a march to the shrine of Imam Ali, which has been taken over by Sadr's militiamen.

U.S. forces breached a road ringing the shrine as they pushed in from two directions and gained footholds in the core of the defenses of Sadr's Mahdi Army, a dramatic move that one commander said could signal "the beginning of the end."

Pockets of militiamen outside the shrine were pounded again by U.S. warplanes and attack helicopters, which unleashed bombs and rockets that fell less than 100 yards from the holy site. Witnesses reported seeing dozens of dead militiamen and said survivors of the air raids were trying to regroup. Although the militia appeared to be at its weakest point since the standoff began on Aug. 5, the witnesses said there were scores of well-armed fighters hiding in the alleys that lead to the shrine.

Sistani's dramatic return posed a potentially significant new complication in the confrontation between security forces and Sadr's militia. Although Sistani has quietly disagreed with Sadr's militant tactics, it is not clear what he wants to accomplish through his march.

Iraqi political leaders expressed concern that the march could be co-opted by Sadr's supporters and that an injection of thousands of noncombatants into the war-torn city could interfere with ongoing military operations and allow the militiamen to escape. But, the political leaders said, it also could reduce tensions by pressuring Sadr to relinquish control of the shrine to more senior Shiite leaders, perhaps leading some fighters to lay down their arms.

Mindful of Sistani's ability to mobilize crowds, Iraq's interim prime minister, Ayad Allawi, flew to Basra late Wednesday to meet with the ayatollah, political sources said. It was not immediately known what the two men discussed.

Mohammed Musawi, a Sistani aide, said the purpose of the march was to save the holy city and shrine. "Americans interfering in this will not help the situation at all," he told the BBC. "We always say that the Americans should be very far from the holy places. They should not involve themselves in this problem."

A spokesman for Sadr said the Mahdi Army would observe a cease-fire along Sistani's route to Najaf on Thursday. "We announce stopping all the operations and fighting in the south and all the provinces which Sistani will pass through, for his sake," Qais Khafaji, the Sadr spokesman in the southern city of Nasiriyah, said in an interview with al-Jazeera television.

Ali Yassiri, a Sadr spokesman in Baghdad, also urged Shiite followers to head to Najaf, a move that could prove risky given the tensions over U.S. military operations so close to the shrine.

"This is a step to show the world that the Iraqis want to end the military operations and try to solve the problem in Najaf peacefully," he said. "We don't want people to go to Najaf to be human shields. We don't want to increase the sufferings of Iraqis."

U.S. commanders immersed in planning and executing a sharp escalation of the battle did not immediately react to the prospect of tens of thousands of Iraqis descending on a war zone. News of Sistani's proclamation reached most field officers in Najaf through indirect routes, such as satellite news channels. A flurry of plans, orders, adjustments and cancellations followed, and in one battalion command center early in the evening, more than a dozen commanders huddled over war plans beside a note -- written in block letters on a legal pad and set aside -- that read: "Al Sistani calls for followers to march to Najaf in next 24 hours."

[After midnight, orders were issued to continue "limited operations" against the militia. Peace talks and cease-fires were widely forecast but not confirmed.

[Early Thursday, an Army cavalry battalion punched across the road ringing the Imam Ali shrine from a third direction. M1-A1 Abrams tanks and Bradley Fighting Vehicles advanced to within 75 yards of the shrine complex before pulling back and setting up a strong point in a structure about 150 yards from the mosque.

[At dawn, enemy fire increased sharply, with rocket-propelled grenades coming from within the walls of the shrine.]

Sistani, the most influential Shiite cleric in Iraq, has shown great deference in the past to U.S. commanders and officials, who have credited him with exercising a calming influence on Iraq's Shiite majority.

Sistani will return to a city barely recognizable as one of the holiest locales in Shiite Islam. Tank, mortar and air bombardment have shattered whole streets and reduced to rubble sections of the neighborhoods adjoining the immediate vicinity of the mosque.

On Wednesday night, soldiers from the U.S. Army's 7th Cavalry Regiment laid siege to a knot of multi-story buildings just inside the road that encircles the maze of alleys and footpaths that immediately surround the shrine. Marines involved in the operation sometimes fought at close quarters, and several officers repeated an account of one Marine who entered a room, found a militiaman holding a rocket-propelled grenade launcher and killed him with a K-bar knife.

A nearby intersection was a horrific site -- a tangle of broken buildings, downed power lines, standing water and jumpy soldiers pivoting one way while tank turrets scanned the other.

"It looks like Sarajevo down there," said Lt. Col. Jim Rainey, commander of the 7th Regiment's 2nd Battalion, referring to the capital of Bosnia, which was severely disfigured during the Balkan wars of the mid-1990s. Rainey added that gunners and pilots have been required to exercise special care around the shrine. "Or rather, the stuff that's supposed to look like Sarajevo looks like Sarajevo. The minarets still look like minarets."

Rainey said the aim was to close the ring road and "destroy Moqtada militiamen."

"The primary way we've been fighting is to make contact with the enemy, fix the enemy and destroy the enemy," he said on Tuesday, adding that militiamen were no longer darting into the open to fire rocket-propelled grenades, only to be gunned down moments later by U.S. forces. "This is the first day we haven't been seeing the enemy coming," he said.

Late Wednesday, four Marine tanks rumbled all the way around a parking garage adjacent to the shrine, at one point passing alongside the west wall of the holy site, according to a senior field officer. "They were by the mosque," the officer said, adding that the Tigers, as the Marine tankers are known, estimated killing or wounding as many as 50 militiamen, including some ferrying ammunition in wheelbarrows.

Najaf's police chief, Ghalib Hashim Jazaeri, said his officers had arrested leaders of the Mahdi Army carrying jewels and other treasures from the shrine. He also said police have uncovered evidence of the presence of foreign guerrillas connected to al Qaeda, but his claim could not be independently verified.

"Some of them escaped carrying Saudi maps and weapons," Jazaeri said. "They are in Najaf now cooperating with the Mahdi Army to kill the citizens and the innocent people by mortars and other weapons."

Al-Jazeera reported that militants loyal to Sadr kidnapped two relatives of Defense Minister Hazim Shalan. The television station broadcast footage of the two kneeling in front of masked men. Al-Jazeera said the group demanded that U.S. forces leave Najaf and that Iraqi police free Ali Smeisim, a Sadr aide who was reportedly arrested on Wednesday.

Najaf's police chief called on citizens in southern Iraqi provinces to wait until the arrival of Sistani before doing anything that might put them in further danger.

In spite of his plea, thousands of people appeared to be walking toward Najaf from Hilla and other points in the south. As about 800 demonstrators from Kufa, many of whom identified themselves as Sadr supporters, reached a U.S. military base outside of Najaf, Iraqi security forces shot at the crowd, killing two and wounding five, witnesses said.

On Wednesday night, policemen from the chief's security detail barged into a hotel in Najaf and arrested more than 50 Iraqi and foreign journalists at gunpoint. The police officers beat some of the reporters and fired assault rifles in the lobby. After the journalists were brought to the main police station, Jazaeri denied they had been arrested and insisted they had simply been summoned for a news conference.

Correspondent Rajiv Chandrasekaran and staff writer Jackie Spinner in Baghdad contributed to this report.

Grand Ayatollah Ali Sistani, seen arriving in Basra, plans to lead a march today to the Imam Ali shrine, which has been taken over by a militia.