American soldiers fought Shiite militiamen in Najaf overnight, and six members of a family, including three children, were killed in the crossfire. At least one militiaman also was killed.
In the northern city of Mosul, a roadside bomb killed four Iraqi policemen on patrol.
U.S. troops and militiamen loyal to radical clerics Muqtada al-Sadr clashed near holy shrines in Karbala, while gunmen killed two journalists from Polish television who were driving from Baghdad to Najaf.
With hundreds of nearby U.S. troops on a mission to capture him, al-Sadr arrived from Najaf at the main mosque in Kufa surrounded by a large number of heavily armed black-garbed gunmen, including at least one carrying an anti-aircraft gun.
"Yes, yes, to freedom! Yes, yes, to independence!," several thousand worshippers chanted as the young renegade cleric delivered a sermon.
In other developments:
U.S. troops have not moved to capture al-Sadr for the past month as he has attended the Kufa prayers. The military has been treading carefully in its confrontation with al-Sadr and his al-Mahdi Army militia, fearing that too much aggression near some of Shiism's holiest shrines will inflame Iraq's Shiite majority.
Still, U.S. forces have intensified their crackdown over the past week, with increasingly bloody clashes in several southern cities. On Friday, large explosions and gunfire were heard near the mosque serving as al-Sadr's headquarters in central Karbala — about 500 yards from two major shrines. The shooting was heard soon before Friday noon prayers.
The gunfire followed hours of clashes in at least three other parts of the city that lasted into the morning. There was no immediate word on casualties.
Al-Sadr's al-Mahdi Army militia was out in force in Kufa and Najaf, fearing a U.S. assault could be imminent. Militiamen and U.S. troops had a fierce mortar exchange overnight.
Fighters — draped with ammunition belts and carrying automatic weapons and rocket-propelled grenade launchers — moved around the streets of both cities in large numbers and took positions behind buildings and earthen mounds.
But U.S. commanders suggested they would stick to their policy of not moving against al-Sadr on Fridays out of respect for the weekly Islamic day of prayer.
Early Friday, four mortar rounds and a rocket-propelled grenade hit near an Italian base in the southern city of Nasiriyah, and two Iraqi police officers were wounded, an Italian spokesman said.
Overnight, a barrage of mortars hit the U.S. base in Najaf, the latest in near nightly shelling. The U.S. troops responded with heavy force, firing back with mortars and howitzers as F-16s overhead directed fire.
The return fire hit a house, killing six family members — including children aged 2, 4 and 5 — and wounding three others.
"It was around midnight and we were preparing to go to bed," said Aziba al-Issa, 28, whose husband, daughter, brother-in-law and nephews were killed.
"The bomb fell on our room," she said from a Kufa hospital bed as she recovered from shrapnel wounds. Her two daughters also were wounded.
The militiamen's mortar position was destroyed, said Maj. Todd Walsh, of the 2nd Battalion, 37th Regiment, 1st Armored Division. The fighters' mortars were becoming more accurate over time, so commanders decided to intensify their response to wipe out the position, Walsh said.
Later, a U.S. convoy of Humvees leaving the Najaf area was ambushed twice in 10 minutes by insurgents firing rocket-propelled grenades and automatic rifles from rooftops. An AP reporter saw five militants apparently hit by U.S. retaliatory fire.