BAGHDAD, Iraq - Two attacks on Iraqi police south of Baghdad on Saturday left four people dead, officers said, in the latest of a string of attacks on local authorities linked to the U.S.-led occupation.

Gunmen fired on a vehicle carrying Col. Wisam Hussein, the police chief of Mahmoudiya, 20 miles south of Baghdad, killing both him and his driver, police said.

The police chief was killed as he was returning home from a trip to the capital, Lt. Ala'a Hussein said.

Later Saturday, six unidentified assailants attacked a four-member police patrol in the town, killing one officer and wounding three others, one seriously, police officer Khaldoon al-Gurairi said. He said a 60-year-old bystander was also killed in the attack.

The killings came a day after a police commander in the southern city of Kufa was shot to death while driving.

Insurgents consider Iraqi police as collaborators with the U.S.-led occupation. On March 24, nine police recruits were killed when gunmen shot at their vehicle in southern Babil province.

More than 350 policemen have been killed by shootings and suicide bombings since the fall of Saddam Hussein's regime last year.

Also Saturday, an explosion near a U.S. military convoy near Khalis, 40 miles northeast of Baghdad, wounded a civilian and damaged a Humvee vehicle, Iraqi officials said. A policeman who witnessed the blast said on condition of anonymity that it came from within a car left on the roadside.

American forces blocked off the area after the attack, witnesses said. It was not clear if any Americans suffered casualties. A U.S. official in Baghdad could not confirm the bombing.

A convoy of Salvadoran soldiers was attacked in the southern city of Najaf late Friday and three troops were wounded, Salvadoran Capt. Roberto Abrigo said. El Salvador has about 380 troops in Iraq, serving as part of the Spanish-led Plus Ultra brigade.

In western Baghdad on Saturday, a rocket slammed into a house in a residential suburb, killing two people and wounding four, said a doctor at Yarmouk hospital, Jamil Ibrahim. It was not clear who owned the house.

Meanwhile, about 5,000 members of radical Shiite Muslim cleric Muqtada al-Sadr's self-styled militia, the al-Mahdi Army, paraded in Sadr City, a mainly Shiite district in eastern Baghdad, on Saturday. The unarmed black-clad militiamen carried portraits of the cleric. They marched past a reviewing stand where Muslim clerics acknowledged their salute.

Hundreds of Iraqis lined the march route. Sharpshooters from the militia were stationed on rooftops. Al-Sadr has been an outspoken critic of the U.S.-led occupation, but has not called for attacking the occupying forces.

A senior U.S. official also said Saturday that investigators were looking at video cassettes shot during the mutilation of the bodies of four American civilians killed Wednesday and were trying to identify people who participated.

The charred remains of the Americans were dragged through the streets for hours after insurgents ambushed their vehicles. Two corpses were hung from a bridge.