Gunmen assassinated an aide to Iraq's most prominent Shiite Muslim cleric on a street in central Baghdad on Friday, and a Sunni Muslim cleric was kidnapped shortly after in the same neighborhood, according to Iraqi security officials.

In the face of these and other sectarian attacks, Muslim preachers used sermons at Friday mosque services to call for an end to violence.

Police said Kamaleddin Ghuraifi, a Baghdad representative of Grand Ayatollah Ali Sistani, was gunned down on his way to Friday prayers at al-Doreen Mosque on Haifa Street, a dangerous area where Iraqi and U.S. officials say order is gradually being restored by Iraqi security forces. Two of Ghuraifi's bodyguards also were killed, according to a spokesman at Sistani's headquarters in Najaf, the Shiite holy city 90 miles south of Baghdad.

"We cannot protect every scholar of Sistani. They number in the thousands," the spokesman said. "We accuse the extremists. This is not the first time they have targeted a Sistani scholar."

Later, gunmen entered a nearby Sunni mosque during Friday prayers and abducted the preacher, Amr Tikriti, police 1st Lt. Mohammed Hiyani told the Associated Press. It was not clear whether the kidnapping was related to Ghuraifi's killing.

Also on Friday morning, a driver detonated a car bomb outside an office of Prime Minister Ibrahim Jafari's Dawa party, one of the principal groups in the Shiite coalition that holds a majority of the seats in Iraq's National Assembly. The attack killed the driver of the car and a bystander.

Conflict between Iraq's majority Shiites and the Sunni Arab minority that wielded power until the fall of President Saddam Hussein in 2003 has flared since Jafari's Shiite-led government was formed in April.

The country's nearly two-year-old insurgency, which is driven by Sunni religious radicals and secular Sunnis loyal to Hussein, has included assassinations of political and religious leaders in the Shiite community as well as bomb attacks on Shiite neighborhoods. Sunni leaders have accused the Shiite-dominated security forces of using security crackdowns to settle sectarian scores.

Since mid-June, Shiites have suffered the brunt of the violence. Another Sistani representative, Samir Baghdadi, was shot dead last week in Baghdad, and on Tuesday, Dhari Fayadh, a Shiite and the oldest member of the National Assembly, was killed by a suicide bomber.

"The sectarian war is escalating, and the objective is to draw the Shiites into civil war," warned Jalaladeen Sagheer, a Shiite cleric, in his Friday sermon at Baghdad's Buratha Mosque. He urged listeners not to use violence, because "if catastrophe struck, it would devastate everything. No one will benefit from it except the enemies of Iraq."In the southern city of Karbala, another Shiite cleric, Mohammad Hussein Almeedy, denounced the insurgents' tactics. "What is the moral and religious justification for killing innocent Iraqis who are Muslims?" Almeedy asked in his sermon. "How can they justify the killing of women, children and old persons?"

Efforts by U.S. and Iraqi military forces to drive insurgents out of towns in western Iraq continued Friday. About 1,000 U.S. Marines, soldiers and sailors and 100 Iraqi troops conducted a fourth day of search operations in the town of Hit, about 95 miles northwest of Baghdad. The Marines said in a statement that the push, called Operation Sword, was continuing to meet little resistance.

Less than a mile north of Hit, the statement said, Marines and sailors found 15 roadside bombs -- 13 of them placed along an 800-yard stretch of road. "The insurgents had attempted to attach several of the bombs together in order to make a larger explosion and increase the blast radius of the bombs," the statement said.

In Ramadi, a city 60 miles west of Baghdad, residents directed police to a spot where the bodies of three men -- described as a Saudi, a Jordanian and a Kuwaiti -- had been dumped out of a pickup truck, according to Iraqi army 1st Lt. Hussein Fadhil Alwan. The residents said the dead men were members of al Qaeda in Iraq, the guerrilla group led by Abu Musab Zarqawi.

Later, a statement posted at a mosque in Ramadi by Zarqawi's group accused a local tribe of the killings and warned: "We promise a fast and very swift punishment for this tribe because of what they did. They are apostates from Islam -- they should be killed."

Special correspondents Saad Sarhan in Karbala and Nasser Nouri and Khalid Alsaffar in Baghdad contributed to this report.

Iraqis carry home buckets of water from a nearby well after a fire at a power station left millions in Baghdad without drinking water.