A sport-utility vehicle filled with explosives detonated Wednesday at the main entrance to the fortified compound in central Baghdad that houses the interim Iraqi government and the U.S. Embassy, killing 11 people, most of them Iraqi civilians, and injuring more than 40 others.

Iraqi officials said the blast, triggered by a suicide bomber, occurred at 9:15 a.m. as many Iraqi civilians waited to pass through a crowded checkpoint leading to the convention center, the site of a number of government offices and booths where American contractors advertise job openings. The force of the explosion rattled hotel windows more than a mile away, collapsed plaster roofs of nearby shops and gouged huge chunks out of the cement blast walls that rim the compound once known as the Green Zone.

Prime Minister Ayad Allawi, who visited the site an hour after the blast, said the dead included four members of the Iraqi National Guard, a U.S.-trained paramilitary force that has assumed a larger role in security operations since the June 28 handover of political power to the interim Iraqi government. Seven Iraqi civilians were also killed, and Iraqi officials at the scene said most of them likely worked inside the compound or were seeking employment through the jobs fair at the convention center.

"These people were here to find work," Allawi said as the wreckage smoldered. "This is naked aggression against the Iraqi people. We will bring them to justice."

The bombing shattered a calm that had settled over the capital in the two weeks since the interim government assumed political authority after 15 months of U.S.-led occupation. It also appeared to be the first time since the handover that a suicide bomber deliberately targeted Iraqi civilians working for the United States.

Insurgents on Wednesday also assassinated the popular governor of Nineveh province, as his convoy traveled from the town of Baiji, 110 miles northwest of Baghdad. Witnesses said attackers fired on the northbound convoy, killing the governor, Osama Youssef Kashmoula, and his deputy. In an ensuing firefight, witnesses said, Kashmoula's security detail killed four insurgents. Kashmoula is believed to be the most senior Iraqi official killed since the May 17 assassination of Izzedin Salim, the acting president of the now-dissolved Iraqi Governing Council. Salim was killed in a suicide car bombing as he waited to enter the Green Zone.

Wednesday's explosion in Baghdad sent plumes of mud-brown smoke over the convention center complex and demonstrated the insurgency's continuing intent and capacity to strike at the political heart of the country. The checkpoint attacked this morning is the primary entrance for pedestrians entering the convention center and is used occasionally by members of the interim government.

The city center was largely empty Wednesday morning, as Iraqis celebrated the anniversary of the 1958 revolution that overturned the country's monarchy. Iraqi police officials at the scene of the blast said there would have been more casualties if the explosion had occurred on a normal business day. Iraqi police Brig. Majid Abdul-Hammed Obeidi estimated that about 1,000 pounds of explosives were packed in the vehicle, which a traffic patrol officer identified as a Toyota Land Cruiser.

U.S. military officials said, however, they had yet to identify the kind of vehicle used in the attack, although Col. John Murray, commander of the 1st Cavalry Division's 3rd Brigade, said it was certainly an SUV of some kind. The driver had been waved into the line to enter the parking lot, Murray said, and the process of searching the vehicle had just begun when he detonated the bomb.

"I wouldn't call it a surprise," Murray said. "These people are getting desperate. I would be surprised if this were the last one."

Judging by the charred, windowless frames of at least five cars, the bomb exploded about 30 feet from the entrance to the parking lot, along a set of 15-foot-high blast walls. The explosion was followed almost immediately by bursts of automatic rifle fire, which Iraqi police said came from confused Iraqi National Guardsmen trying to gain control over the scene.

Given the symbolic importance of the anniversary, U.S. forces had been placed on heightened alert at checkpoints and at other potential targets, including oil production facilities and police stations. But U.S. officials acknowledged that even the increased vigilance, which entailed additional troops and more-stringent search procedures at the checkpoints, could not thwart a determined suicide bomber.

"There is no place in this country where it would be possible to prevent someone willing to give up their life to detonate a bomb," said Lt. Col. Robert Campbell, commander of the 1st Cavalry Division battalion responsible for the six checkpoints into the International Zone, as the Green Zone is now officially known.

Allawi, who visited the site with Deputy Prime Minister Barham Salih, said the attack may have been a reprisal for the aggressive steps he has taken against the insurgency since taking office. In recent days, Iraqi security forces have rounded up more than 500 suspected criminals and insurgents.

"We think this is a response to the recent arrests," Allawi said. "We are going to defeat the evil forces. We are going to build a democratic Iraq."

Allawi described three of those arrested as leaders of the insurgency, including one who he said was not an Iraqi citizen. Speaking to reporters later in the day, the interim president, Ghazi Yawar, suggested the bomber might not have been Iraqi, a common assertion among Iraqi officials bewildered by the insurgents' targeting of civilians. U.S. military officials have acknowledged that, while bringing money and skill to the insurgency, foreign militants likely make up a small fraction of the overall resistance.

Yawar contended that in Iraq's history "there have not been problems between the Iraqis, be they ethnic or religious."

"These people are like a cancer . . . that plagues the Iraqi body," Yawar said of the insurgents. "We will treat it the proper way, God willing. We ask our people . . . for patience and, God willing, they will see results soon."

"All of the people waiting there in line were 100 percent Iraqis. There were no foreigners," said Sgt. Hussein Jassim, 23, of the Iraqi police, who helped carry 11 victims to the hospital in his police pickup truck. The truck's flat bed, as well as Jassim's pale-blue uniform shirt, were smeared with blood. He pointed to the stained Iraqi police patch on his shirt and noted plaintively that it was "filled with Iraqi blood."

"They said in the beginning these operations were against the occupiers because there is occupation, and they said let the Iraqis take over. So why are they doing this now?" Jassim said. "If they keep doing this, the occupiers will say the Iraqis can't keep the country safe and they will stay more. I don't understand. Do they want them to stay more?"

Two hours after the blast, Salem Baqir, 59, was being treated at Yarmouk Hospital for wounds to his back, neck and side. He and his wife, Zahra Kadhim Abbas, had planned to enter the compound to seek an exemption from U.S.-imposed rules that bar members of the once-ruling Baath Party from holding government jobs. As they crossed the intersection, the blast knocked them down.

"I don't know what happened to her," said Baqir, a retired teacher. "The last thing I saw, she was on the ground bleeding. I pray to God for her safety."

Special correspondents Naseer Nouri, Bassam Sabti and Khalid Saffar contributed to this report.

A U.S. soldier runs to scene of a bombing near Baghdad's convention center, which houses government offices.An Iraqi woman at a Baghdad hospital comforts her husband, one of more than 40 injured by a car bomb.