KUFA, Iraq (AP) -- A mortar barrage slammed into a mosque filled with Iraqis preparing to march on the embattled city of Najaf, killing 27 people and wounding 63 here today, hours before the nation's top Shiite cleric arrived at the gates of the holy city in a massive convoy hoping to end three weeks of fighting.
"We are now in Najaf," said Hamed al-Khafaf, an assistant to the Grand Ayatollah Ali Husseini al-Sistani.
In Kufa, Najaf's twin city, unidentified gunmen opened fire from an Iraqi National Guard base on thousands of Shiite Muslim marchers heading to Najaf, killing at least three and wounding 46, witnesses said.
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 dampened renewed hopes for a rapid resolution to the three-week crisis that has pitted al-Sadr loyalists against a combined U.S.-Iraqi force. The U.S. military and the insurgents both blamed the other for the mortar barrage on Kufa's main mosque.
Al-Sistani, 75, returned to Iraq on Wednesday from a trip to London. Traveling in an armored car, al-Sistani left the southern city of Basra in a 30-vehicle convoy of sport utility vehicles earlier today.
The cleric wields enormous influence among Shiite Iraqis, and his return could play a crucial role in stabilizing the Najaf crisis, which has worsened in his absence.
Najaf Gov. Adnan al-Zurufi declared a 24 hour cease-fire from the time al-Sistani arrives in Najaf to give the militants a chance to decide whether to accept the cleric's peace plan.
"Immediately after that, the operation will start again if the militia disagree on what Ayatollah al-Sistani said," he said.
He also said that Iraqi and U.S. forces would temporarily pull back from positions near the revered Imam Ali Shrine to allow for peace efforts.
Al-Sistani's aides said thousands of chanting supporters ran out to greet the convoy as it passed through the town of Samawah.
"It seems that there are huge crowds joining the march in every city we pass by," al-Khafaf said. The convoy was joined by at least a thousand cars, apparently filled with Shiite supporters heeding a call al-Sistani's aides made a day earlier to march on Najaf to stop the violence.
Al-Sistani had called for Iraqis to march to Najaf today, but al-Khafaf said the demonstrators should stop on the edge of the holy city and wait for al-Sistani to arrive and give them instructions before entering.
"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."
In Kufa, Hussam al-Husseini, an al-Sadr aide, said one mortar shell hit the mosque compound itself and two others hit near the mosque gates. Others gave conflicting accounts of the number of explosions.
Blood was splattered on the pavement in a courtyard beside the mosque and a pair of sandals was left nearby, according to Associated Press Television News footage. Shrapnel from the explosions tore small chunks out of walls and the pavement, but the compound did not appear to have suffered serious structural damage.
Mohammed Abdul Kadhim, an official at Furat al-Awsat hospital in Kufa, said 27 people were killed and 63 were injured. The morgue overflowed with bodies, and more than a dozen corpses had to be stored in the hospital's garden.
Outside the hospital's gate, crowds of angry people gathered, shouting "God is great!"
Al-Sadr has regularly delivered a sermon during Friday prayers at the Kufa mosque.
After the attack, thousands of demonstrators loyal to al-Sadr marched on nearby Najaf but came under fire from a base between the two cities housing Iraqi national guardsmen and U.S. troops, witnesses said.
The marchers scattered when the gunfire broke out. The day before, gunfire from the same base killed eight people and wounded 56 others who were taking part in what appeared to be a peaceful demonstration supporting al-Sadr.
Al-Sadr's aide, al-Husseini, said another mosque in Kufa had also been hit by mortar rounds today, but it was unclear whether casualties were reported.
He blamed the mortar attack on the main mosque on American forces backing Iraqi troops in the city, but a U.S. military spokesman denied that.
"We held the interim government responsible for this bombing. We hold it responsible for this bloodshed," al-Husseini said.
Any damage inflicted by U.S. forces on holy sites would anger Iraq's Shiite majority and could spark a greater uprising against the fledgling interim government, which is also battling a persistent and bloody Sunni insurgency.
Marine Capt. Carrie Batson said in an e-mail to The Associated Press today that U.S. forces in the area "continue not to target holy sites, including the Kufa mosque."
In other violence, saboteurs attacked about 20 oil pipelines in southern Iraq late Wednesday, reducing exports from the key oil producing region by at least one-third, an official with the state-run South Oil Co. said today on condition of anonymity.