Baghdad, Iraq ? Iraq’s paramount Shiite cleric unexpectedly returned to Iraq on Wednesday and thrust himself into the bloody standoff in Najaf, calling for a nationwide march on the holy city in a bid to quell three weeks of fighting between U.S. forces and Shiite rebels.

The call for peace from Grand Ayatollah Ali al-Sistani — whose words have sent thousands into the streets in the past — could be a pivotal turn in the showdown between rebel cleric Muqtada al-Sadr and the U.S.-backed Iraqi government.

It raises hopes among some Iraqis of a breakthrough, but also sets the stage for a huge and potentially volatile human surge into a city swarming with U.S. troops and Iraqi militants.

Al-Sistani, 73, who left Iraq Aug. 6 for heart surgery in London, arrived in southern Iraq, crossing from Kuwait. He returns at a moment of soaring tension: U.S. troops on Wednesday took control of virtually the entire center of war-scarred Najaf, closing in around the ancient shrine where al-Sadr’s rebels have holed up with civilian supporters, in the hope that U.S. forces will not assault the holy site. Iraqi government leaders added to already dire warnings that a final clash could be imminent unless al-Sadr backs down.

“The Mahdi Army is finished,” said Najaf’s police chief, Maj. Gen. Ghalib al-Jazaari. “Its hours are numbered.”

Al-Sadr hasn’t been seen in days and, as rumors fly that he has fled the city, insurgents have been seen departing Najaf as well.

It’s not immediately clear how each side may seek to capitalize on al-Sistani’s intervention; the ayatollah did not immediately express support for either side or signal how he hopes to settle the conflict.

In earlier battles between al-Sadr and authorities, al-Sistani has applied blame equally to both sides, condemning the use of U.S. firepower in the holy city, while his representatives criticize al-Sadr for attacking American forces and fueling bloodshed.

Without addressing the details, al-Sadr’s militants swiftly endorsed al-Sistani’s intervention, calling for a cease-fire in every region of the country he passes through on his way from the southern border to his home city of Najaf.

Al-Sadr responded to al-Sistani’s call with a command of his own, urging his followers throughout Iraq to join in the journey to Najaf, adding to a potential migration of thousands, if supporters turn out in numbers comparable to previous Shiite demonstrations.

By nightfall Wednesday, some al-Sistani supporters had already packed into cars and set off for Najaf and Kufa. Others in Baghdad said they were preparing to make the trip.

“I’m ready to go to Najaf with my friends in my own car. … Somebody should save the shrine,” said Ahmed Jassim Mohammed, 27, a photographer in the capital’s Sadr City neighborhood. “The Imam Ali shrine is not just a building to us. I reject the presence of the gunmen inside the shrine. Both sides should respect the holiness of the shrine.”

Smoke billowed over the squat, drab buildings of the Old City in Wednesday’s television images, as machine-gun and mortar fire filled the air.

Najaf police also announced the arrest Wednesday of several al-Sadr aides, including top spokesman Sheik Ali Smeisim, who authorities accused of possessing precious artifacts from the ancient shrine. At a news conference, police showed an ancient carved mud tile from the shrine that they said Smeisim had with him at the time of his capture.

In separate violence Wednesday, gunmen in Kufa killed two protesters in an otherwise peaceful pro-al-Sadr demonstration, The Associated Press reported. The gunfire broke out as hundreds of demonstrators shouted denunciations of the interim Iraqi government, while the parade route wound near an Iraqi National Guard post.

In the western city of Fallujah, meanwhile, sustained U.S. bombing of suspected insurgent gun positions killed four people and injured four others, AP said.