NAJAF, Iraq - Iraq's top Shiite cleric made a dramatic return to Najaf at the head of a massive convoy Thursday hoping to end three weeks of fighting in the city, but hours earlier a mortar barrage slammed into a mosque filled with Iraqis preparing to join his march, killing 27 people and wounding 63.

Along with the mortar attack, another group of thousands of marchers heading into Najaf from its sister-city Kufa came under fire from an Iraqi National Guard base. At least three people were killed and 46 wounded.

Fierce clashes also continued today in Najaf with U.S. warplanes bombing suspected positions of radical Shiite cleric Muqtada al-Sadr's Mahdi Army militia and explosions booming across the city.

The violence could undermine the attempt by Grand Ayatollah Ali al-Husseini al-Sistani - Iraq's most widely respected Shiite cleric - to end the fighting between U.S.-Iraqi forces and al-Sadr's Mahdi Army.

The 75-year-old al-Sistani had been in London undergoing medical treatment throughout the fighting, and his return could be the best hope so far of ending the violence, which has repeatedly seen peace efforts fall apart.

Al-Sistani has held himself above the fray, supporting neither al-Sadr nor the U.S. troops and the pro-U.S. government. He holds the loyalty of a far broader swath of Iraq's Shiite majority than al-Sadr, a young cleric whose fiery anti-U.S. message has drawn many poorer, disillusioned Shiites but is seen by other Shiites as too radical.

Al-Sistani is calling for Najaf and Kufa to be declared weapons-free cities, for all foreign forces to withdraw from Najaf and leave security to the police and for the Iraqi government to compensate those harmed by the fighting here.

Al-Sistani's 30-vehicle convoy drove 220 miles from the southern city of Basra to Najaf, joined by at least a thousand cars from towns along the way, where supporters on the street cheered al-Sistani.

He arrived in Najaf just before 3 p.m. and went directly to one of his houses in the al-Sa'ad neighborhood, about a mile from the revered Imam Ali Shrine, where the militants were holed up.

Interim Prime Minister Ayad Allawi declared a 24-hour cease-fire in Najaf from the time of al-Sistani's arrival "to reinforce our commitment to peace."

Allawi expressed hope al-Sistani's peace initiative would succeed so the government would not have to resort to a long-threatened raid on the Imam Ali Shrine - an action that would likely outrage Iraq's Shiite majority.

"I stress that this is the last call for peace and that this is the last chance to put an end to the spilling of innocent blood," Allawi said in a statement. "God willing, our prayers for Iraq's peace and stable security will be met."

Najaf Gov. Adnan al-Zurufi said Iraqi and U.S. forces would temporarily pull back from positions near the revered Imam Ali Shrine to allow for peace efforts.

Allawi and other government officials have repeatedly issued ultimatums for the militants to capitulate, but have yet to follow through on their threats.

Al-Sistani's aides have called on Iraqi Shiites nationwide to march on Najaf to support his peace bid, but asked them to wait on the edge of the holy city until his arrival. Al-Sadr's aides also asked their followers to join the march as well.

With officials from all sides supporting al-Sistani's march call, it was unclear who fired the mortars that hit the main mosque in Kufa, where thousands of Shiites had gathered intending to march to nearby Najaf.

Al-Sadr aides blamed U.S. forces, but American commanders denied their troops fired any mortars. Al-Sadr regularly preaches at the mosque, which has long been one of the centers of his support.

"We were gathering outside and inside the mosque preparing to head to Najaf when two mortar shells landed, one inside the mosque and the other on the main gate," said Hani Hashem, bringing an injured friend to the hospital. "This is a criminal act. We just wanted to launch a peaceful demonstration."