There were conflicting reports about which facility was the target. Iraqi officials said Friday's attack was against the Interior Ministry lock-up where US troops recently found detainees who appeared to have been tortured.
But US troops said the target appeared to be the al-Hamra Hotel, another heavily fortified compound where foreign news agencies are based. There were no immediate reports of foreign casualties in the 8.20am (0520 GMT) blast.
The double bombings in the Jadiriya neighbourhood reverberated through the city centre, sent a mushroom cloud hundreds of feet into the air and was followed by sporadic small-arms fire.
Major Falah al-Mohammedawi from the Interior Ministry said he believed that Abu Musab al-Zarqawi's al-Qaida in Iraq was behind the bombing.
"The attacks were targeting the [Interior Ministry building] itself and we believe that al-Zarqawi's group is behind the blasts as revenge for the fact that some al-Qaida members were inside the shelter," he said.
Associated Press Television News footage showed that several residential buildings had collapsed from the blast and a large crater in the road.
But US soldiers said the attack was a copycat of a car bombing against journalists in the Palestine Hotel last month, with the first car bomb attempting to knock down the hotel's defensive wall and the second vehicle trying to penetrate the breach.
Defence workers tried to pull
victims out of the rubble
"They tried to knock down the wall to get to the hotel," said Sergeant-Major Stanley, who did not give his first name.
"There are a lot of casualties and a lot of damage."
Deputy Interior Minister Major-General Ali Ghalib said the Interior Ministry building was very close to the al-Hamra Hotel but he believed that the hotel was the target.
"The hotel is fortified and the shelter is fortified," Ghalib said. "It was random explosions because the suicide attacker could not reach the target and that is the reason why the damage was in a civilian building.
"I think that the hotel was targeted because foreigners usually stay there," Ghalib said.
Engineering units from the US army were sent to the scene to help in the rescue effort, a statement from the US 3rd Infantry Division said.
Victims pulled out
Firefighters joined neighbours to dig through the debris and pull victims out.
At least one family was believed buried in the rubble, al-Mohammedawi said.
"We are trying to rescue them and we hope to find them alive," he said.
"The attacks were targeting the [Interior Ministry building] itself and we believe that al-Zarqawi's group is behind the blasts as revenge for the fact that some of al-Qaida members were inside the shelter"
Major Falah al-Mohammedawi
Police-Captain Nabil Abdel-Qadir said the two car bombs were detonated behind the Interior Ministry building.
Six civilians were killed and three police officers were among the injured, he said.
A five-member family was rescued by firefighters after part of their house collapsed, police Major Raed Abbas-Salman said.
The mother had serious burns because she was in the kitchen, while the father and three children suffered shrapnel wounds.
US troops found up to 173 malnourished detainees, some showing signs of torture, in the building on Sunday. Most were believed to be Sunni Arabs.
A leader of a major Sunni party, Tariq al-Hashimi, told Iraq's Sharqiya television on Thursday that his group had submitted 50 complaints of prisoner abuse to the government.
"We did not receive a timely response," he said.
However, Interior Minister Bayan Jabir, a Shia, brushed aside the complaints, denied sectarian bias and claimed that "every time" al-Hashimi has differences with him "he exerts pressure on me through the US embassy".
Six civilians were killed and three
police officers injured in the blast
"I reject torture and I will punish those who perform torture," Jabir said. "No one was beheaded, no one was killed.
"Those who are supporting terrorism are making the exaggerations," he said, adding that only seven detainees showed signs of abuse.
In a statement on Thursday, the US embassy said the Iraqi authorities had given assurances that they would investigate the conditions of detainees found on Sunday night. The statement said the abuse of prisoners would not be tolerated by either the Iraqi government or US-led forces anywhere in the country.
"We have made clear to the Iraqi government that there must not be militia or sectarian control or direction of Iraqi security forces, facilities or ministries," the US statement added.
Prominent Sunni Arabs have complained for months about abuse by Interior Ministry forces, whom they claim have been infiltrated by Shia militias. The Sunnis called for an international investigation after the Jadriya detainees were found.
The government denies the militia allegations.