The bomb struck a convoy carrying the Khalil Jalil Hamza, the governor of the Qadisiyah province, and the provincial police chief home from a funeral service for a tribal sheik at about 5 p.m., army Brig. Gen. Othman al-Farood said.
Hamza and the police chief, Maj. Gen. Khalid Hassan, were killed, along with their driver and a body guard who were in the same SUV, according to al-Farood, the commander of the Iraqi army division in charge of the area.
The attack occurred in the town of Aajaf, as the convoy was headed back to the provincial capital of Diwaniyah, 80 miles south of Baghdad.
Diwaniyah has been the site of heavy clashes between U.S.-Iraqi security forces and Shiite militia fighters. The area also has seen a rise in internal rivalries between rival militia forces, including the Mahdi Army that is loyal to radical Shiite cleric Muqtada al-Sadr.
Authorities imposed an indefinite curfew on the city.
In Baghdad, militants bombed the house of a prominent anti-al Qaeda Sunni cleric, seriously wounding him and killing three of his relatives in what appeared to be an increased campaign against Sunnis who have turned against the terror network.
That attack, which was followed by a fierce firefight, came after Sheik Wathiq al-Obeidi called on residents in the northern Baghdad neighborhood of Azamiyah to rise up against foreign fighters, a reference to al Qaeda in Iraq, which recently has seen a surge in opposition from fellow Sunnis.
A Sunni insurgent umbrella group threatened the cleric on Tuesday, calling him a traitor and accusing him of working with the Anbar Salvation Council, an alliance of Sunni tribal leaders who are fighting al Qaeda in Iraq in the province of the same name west of Baghdad.
"The so-called Wathiq and his followers ... are a legitimate target for mujahedeen (holy warriors)," the statement said.
Followers denied the cleric, a former preacher at the Abu Hanifa mosque, was linked to the U.S.-backed Anbar group.
But he issued his own call against al Qaeda in Iraq last week during a funeral prayer for two nephews killed by militants believed to be linked to the group.
"We have to fight foreign fighters in our city," witnesses quoted him as saying. "We have to fight those linked to al Qaeda in Azamiyah."
The explosion struck al-Obeidi's house before dawn and was followed by gunfire that resounded across the predominantly Sunni neighborhood.
The cleric, a former preacher at the Abu Hanifa mosque, was seriously wounded and his brother and two female relatives were killed, according to the head of the neighborhood council Dawood al-Azami.
Azamiyah is one of the most dangerous neighborhoods in Baghdad and has been surrounded by a security barrier as the U.S. and Iraqi militaries try to assert control over the area.
The U.S. attorney general who is under fire at home with calls for his resignation, meanwhile, arrived in Baghdad for his third trip to Iraq to meet with department officials who have been there to help fashion the country's legal system.
"I am pleased to see firsthand ... the progress that the men and women of the Justice Department have made to rebuild Iraq's legal system and law enforcement infrastructure," Alberto Gonzales said in a statement released by the Justice Department.
Gonzales got an update from Gen. David Petraeus, the top U.S. commander in Iraq, and also planned to meet with Ryan Crocker, the U.S. ambassador to Iraq, and other U.S. and Iraqi officials, the statement said.
Gonzales was an architect of U.S. policy on the treatment of prisoners abroad and author of a 2002 memo saying the president had the right to waive laws and treaties that protect war prisoners. U.S. President George W. Bush has staunchly defended the attorney general.
On the political front, Shiites and Sunnis had mixed reactions to the international community's decision to expand the U.N. role in Iraq and open the door for the world body to promote talks to ease Iraq's sectarian bloodshed.
The resolution adopted Friday by the Security Council authorizes the United Nations — at the request of the Iraqi government — to promote political talks among Iraqis and a regional dialogue on issues including border security, energy and refugees as well as help tackling the country's worsening humanitarian crisis which has spilled into neighboring countries.
"The U.N. is a neutral party that can play a good role in Iraq. They have played good role previously and now, we need them to re-activate that role and expand it, so we welcome this renewed chance for them here in Iraq," said Salim Abdullah, a spokesman for the Iraqi Accordance Front, the largest Sunni political bloc in parliament.
"Finding a third party, however, does not lift the responsibility from the shoulders of the American administration," he added. "It should be clear for the political powers inside Iraq that they cannot completely rely on the U.N., which should have a complementary role."
Independent Shiite politician Qassim Dawoud, however, worried the move was a precursor to a withdrawal of much-needed U.S. support for the country.
"When the Americans move the ball toward the U.N. court, it means the beginning of abandoning these commitments," he said. "We welcome any activation of the U.N. role on condition that the United States does not abandon its commitments."
Separately, the U.S. military on Saturday reported the death of a Task Force Lightning soldier in a non-combat incident.
In other violence reported by police:
— The bodies of four men abducted a week ago were found chopped into pieces in Dujail, 80 kilometers (50 miles) north of Baghdad.
— A roadside bomb killed one civilian and wounded another while they were driving on the highway south of Baghdad.
— A police patrol was struck by a roadside bomb in northern Iraq, killing one officer and wounding two others.
The officials who reported the violence spoke on condition of anonymity because they were not authorized to release the information.