Another suicide car bomber later blew himself up in front of the Slovakian Embassy in southeast Baghdad, injuring four people.
Attacks by suicide bombers and gunmen in Baghdad and south of the capital left at least 23 dead.
Just south of Baghdad, gunmen killed at least 11 Iraqi construction workers and injured another three when they riddled their minibus with bullets as it drove in the so-called Triangle of Death, police said. The attack occurred in Diyara, 30 miles south of Baghdad, and the men worked on construction projects on Iraq and American bases, police added.
In other recent developments:
In Baghdad, gunmen also attacked in a speeding car attacked another Interior Ministry commando convoy in western Baghdad's Mansour area, killing three commandos, police said.
The attack at the two-story headquarters of the Wolf Brigade follows weeks of accusations against the Shiite Muslim-dominated force by Sunni Arab leaders, who accuse it of kidnapping and killing Sunnis, including clerics.
Interior Minister Bayan Jabr said the attacker was a former Wolf Brigade member who was targeting the commando force's commander, Brig. Mohammed al-Quraishi.
"Today's attack does not constitute an infiltration of the police forces," Jabr said during a press conference. "The only thing left of the bomber was his head and feet."
Jabr said three people were killed in the blast. But a witness to the attack, Maj. Falah al-Mahamdawi, said five people were killed and seven wounded. It was not immediately possible to reconcile the discrepancy. Police are hunting for two of his former colleagues, the minister added.
It was unclear how the attacker managed to enter the tightly guarded compound in eastern Baghdad's Bab Sharqi neighborhood with his explosives being undetected. But by being dressed in police uniform, the attacker may have managed to avoid the stringent checks in place.
People entering the compound, which also houses the 10-story Interior Ministry building, must go through metal detectors and be frisked by policemen and checked by sniffer dogs before arriving at the building.
Al-Mahamdawi, the police officer, said the attacker was disguised as a policeman and detonated explosives during a roll call for recently graduated commandos at the Wold Brigade headquarters.
"I was inside the headquarters building when the explosion took place," al-Mahamdawi said. "Then I saw five dead bodies lying on the ground plus seven injured people, most of them are policemen."
"There was a group of newly graduated commandos gathered in the yard at the time of the explosion," he added. It was unclear if the bomber was standing among the graduates or was nearby.
The attack may have been motivated by the Sunni struggle for a role in the country's political process. Sunnis, a minority favored by Saddam Hussein, resent the rise to power of the majority Shiite community and the U.S.-allied Kurds.
This is believed to be a major factor in the continuing insurgency in Iraq that has killed at least 934 people since the new Shiite-led government was announced April 28.
Jabr said that an ongoing counterinsurgency campaign to weed out militants in Baghdad had led to 1,318 arrests and claimed it was showing some success in drastically reducing the number of car bomb attacks in the capital.
"Operation Lightning has forced the terrorists to flee outside Baghdad," Jabr told a news conference.
He said that before the campaign, which will enter its third week Sunday, there were an average of 12 car bombings in Baghdad and had now dropped to 0.6 percent a day. He added that more than $6 million was confiscated and that eight foreigners — all Arabs — were among those captured.
He added that eight people had been arrested in connection witha Friday car bombing outside a Baghdad falafel restaurant that killed 10 people, including two children.
Iraqi police also raided an auto mechanic's workshop in the southeastern New Baghdad suburb and captured five men suspected of rigging car bombs, Lt. Col. Ahmed Abood said. It was unclear which attacks the men have allegedly been involved in.
Two Oil Ministry employees were shot dead early Saturday in Baghdad's southern Dora district, police official Rasol Salih said. A third man was critically wounded and taken to hospital.
The bodies were found in a canal under a footbridge and one of the slain men was seen handcuffed and wearing civilian clothes, according to footage taken by Associated Press Television News.