At least three of the dead were Iraqi members of Parliament, reports CBS News correspondent Martin Seemungal.
A news video camera captured the moment of the blast: a flash and an orange ball of fire causing a startled parliament member who was being interviewed to duck, and then the smoky, dust-filled aftermath of confusion and shouting. The video was shot by Alhurra, a U.S. government-funded Arab-language channel.
Iraqi officials later gave wildly varying accounts of how many people were killed and who they were. Some disputed the U.S. death toll but gave no definitive figure of their own.
Seemungal reports that an Iraqi member of parliament who was inside the cafeteria when the explosion occurred said as many as six lawmakers may have been killed by the blast.
The explosion came hours after a suicide truck bomb exploded on a major bridge in Baghdad, collapsing the steel structure and sending cars tumbling into the Tigris River, police and witnesses said. At least 10 people were killed.
The parliament bombing was believed to be the deadliest attack in the Green Zone, the enclave that houses Iraq's leadership as well as the U.S. Embassy, and is secured by American and Iraqi checkpoints.
Security officials at parliament, who spoke on condition of anonymity because they were not authorized to release the information, said they believed the bomber was a bodyguard of a Sunni member of parliament who was not among the dead. They would not name the member of parliament.
The officials also said two satchel bombs were found near the cafeteria. A U.S. bomb squad took the explosives away and detonated them without incident.
President Bush strongly condemned the attack, saying: "My message to the Iraqi government is 'We stand with you.'"
White House spokesman Scott Stanzel said earlier that the attack shows "terrorists and extremists will go to great lengths to undermine the Iraqi government," reports CBS News correspondent Peter Maer.
"The U.S. and Iraq cannot and will not let them succeed," Stanzel added.
Stanzel said U.S. and other coalition forces would take steps to "strengthen security."
"We've known there's a security problem in Baghdad, which is why the president has structured a new strategy and why Gen. (David) Petraeus and his commanders are carrying it out," added Secretary of State Condoleezza Rice. "But this is still early in the process and I don't think anybody expected there would not be counter-efforts by terrorists to undermine the security progress we're trying to make."
Maj. Gen. William Caldwell told The Associated Press that eight people were killed in the attack.
Iraqi officials said the bomber struck the cafeteria while several lawmakers were eating lunch, and at least two of them — both Sunnis — were killed. State television said 30 people were wounded.
"We don't know at this point who it was. We do know in the past that suicide vests have been used predominantly by al Qaeda," Caldwell said.
Government spokesman Ali al-Dabbagh suggested that those behind the attack might work in the building.
"There are some groups that work in politics during the day and do things other than politics at night," he told Alhurra.
The Alhurra video showed the blast, with startled lawmaker Jalaluddin al-Saghir, who is also a Muslim imam, ducking for cover. It then showed the immediate aftermath: People screamed for help in a smoky hallway, with one man slumped over, covered in dust and motionless. A woman kneeled over what appeared to be a wounded or dead man near a table. The camera then focused on a bloody, severed leg.
TV cameras and videotapes belonging to a crew sending footage to Western networks were confiscated and apparently handed over to U.S. authorities.
After the blast, security guards sealed the building and no one—including lawmakers — was allowed to enter or leave.
A spokesman for the U.S. Embassy in Baghdad said no Americans were hurt.
The bombing came amid the two-month-old security crackdown in Baghdad, which has sought to restore stability in the capital so that the government of Iraq can take key political steps by June 30 or face a withdrawal of American support.
In other developments: