BAGHDAD — They protect American diplomats, guard supply convoys, and perform dangerous tasks for a military stretched to the limit by the conflict in Iraq. But the presence of armed civilians operating outside both American military and Iraqi law has long angered Iraqis.

That anger exploded yesterday when the Interior Ministry announced that it had revoked the license of Blackwater USA after what it said was the fatal shooting of eight Iraqi civilians following a car bomb attack against a State Department convoy.

The order by the Interior Ministry, if carried out, would deal a severe blow to American government operations in Iraq by stripping diplomats, engineers, reconstruction officials, and others of their security protection.

The presence of so many visible, aggressive Western security contractors has angered many Iraqis, who consider them a mercenary force that runs roughshod over people in their own country.

Sunday’s shooting was the latest in a series of incidents in which Blackwater and other foreign contractors have been accused of shooting to death Iraqi citizens. None has faced charges or prosecution.

Secretary of State Rice telephoned Prime Minister al-Maliki late yesterday, and the two agreed to conduct a “fair and transparent investigation” and hold any wrongdoers accountable, said Yassin Majid, an adviser to the prime minister. Ms. Rice was expected to visit the Middle East today.

Mr. Majid made no mention of the order to expel Blackwater, and it was unlikely that America would agree to abandon a security company that plays such a critical role in American operations in Iraq.

A State Department official confirmed the call but said he could not describe the substance. America clearly hoped the Iraqis would be satisfied with an investigation, a finding of responsibility, and compensation to the victims’ families — and not insist on expelling a company that the Americans cannot operate here without.

Details of Sunday’s incident were unclear.

An Interior Ministry spokesman, Abdul-Karim Khalaf, said eight civilians were killed and 13 were wounded when contractors believed to be working for Blackwater USA opened fire on civilians in the predominantly Sunni neighborhood of Mansour in western Baghdad.

“We have canceled the license of Blackwater and prevented them from working all over Iraqi territory. We will also refer those involved to Iraqi judicial authorities,” Mr. Khalaf said.

He said witness reports pointed to Blackwater involvement but added that the shooting was still under investigation. One witness, Hussein Abdul-Abbas, said the explosion was followed by about 20 minutes of heavy gunfire and “everybody in the street started to flee immediately.”

American officials said the motorcade was traveling through Nisoor Square on the way back to the Green Zone when the car-bomb exploded, followed by volleys of small-arms fire that disabled one of the vehicles but caused no American casualties.

According to TIME.com, which obtained an American incident report, a separate convoy arriving to help was “blocked/surrounded by several Iraqi police and Iraqi national guard vehicles and armed personnel.”

Blackwater said the company had not been formally notified of any expulsion.

“Blackwater’s independent contractors acted lawfully and appropriately in response to a hostile attack in Baghdad on Sunday,” a spokeswoman, Anne Tyrrell, said in a statement late yesterday.

“The ‘civilians’ reportedly fired upon by Blackwater professionals were in fact armed enemies and Blackwater personnel returned defensive fire,” she said. “Blackwater regrets any loss of life but this convoy was violently attacked by armed insurgents, not civilians, and our people did their job to defend human life.”

American officials refused to explain the legal authority under which Blackwater operates in Iraq or say whether the company was complying with the order. It also was unclear whether the contractors involved in the shooting were still in Iraq.

The incident drew attention to the use of heavily armed private security contractors who Iraqis complain operate beyond the control of American military and Iraqi law.

The events in Mansour also illustrate the challenge of trying to protect American officials in a city where car bombs can explode at any time, and where gunmen blend in with the civilian population.

“The Blackwater guys are not fools. If they were gunning down people, it was because they felt it was the beginning of an ambush,” an independent military analyst and author of the book “Licensed to Kill,” Robert Young Pelton, said “They’re famous for being very aggressive. They use their machine guns like car horns. But it’s not the goal to kill people.”

In one of the most horrific attacks of the war, four Blackwater employees were ambushed and killed in Fallujah in 2004 and their charred bodies hung from a bridge over the Euphrates River.

But Iraqis have long complained about high-profile, heavily armed security vehicles careering through the streets, with guards pointing weapons at civilians and sometimes firing warning shots at anyone deemed too close. And Iraqi officials were quick to condemn the foreign guards.

Mr. Maliki late Sunday condemned the shooting by a “foreign security company” and called it a “crime.”

Interior Minister Jawad al-Bolani described the shooting as “a crime about which we cannot be silent.”

“Everyone should understand that whoever wants good relations with Iraq should respect Iraqis,” Mr. Bolani told Al-Arabiya television. “We are implementing the law and abide by laws, and others should respect these laws and respect the sovereignty and independence of Iraqis in their country.”

Defense Minister Abdul-Qadir al-Obaidi told Iraqi television that “those criminals” responsible for deaths “should be punished” and that the government would demand compensation for the victims’ families.