A driver set off a truck bomb at the gates of the U.S.-led coalition headquarters Sunday, killing about 20 people and wounding 63 in the deadliest attack here since Saddam Hussein's (search) capture last month.

The bombing, which occurred during rush hour on a chill foggy morning, came on the eve of a meeting between U.S. administrator L. Paul Bremer (search) and U.N. Secretary-General Kofi Annan (search) to discuss Iraq's future, including whether Iraq is safe enough for the world body to return.

Witnesses said that at about 8 a.m., the driver of what the U.S. military described as a white Toyota pickup truck tried to bypass a line of Iraqi workers and a crowd of U.S. military vehicles, coming as close as possible to the entrance American troops call "Assassins' Gate."

The attack was the first vehicle bombing inside the U.S.-controlled "Green Zone" along the west bank of the Tigris River (search), which insurgents have targeted in the past with mortar and rocket fire. The force of the blast, from a bomb containing 1,000 pounds of explosive, rattled windows more than a mile away.

Most victims were Iraqis, waiting in cars or lined up for stringent security checks before going to work or attending other business inside the high-walled coalition compound, housed in what was once Saddam's Republican Palace. The blast occurred during rush hour on Sunday, which is a work day in Iraq.

Mohammed Jabbar, who works at the Ministry of Planning, said the blast "lifted us into the air" and people "fell on top of one another."

Several cars caught fire. Charred metal remains of the truck were hurled hundreds of yards away. Thick black smoke merged with the milky morning fog as armed U.S. soldiers tried to keep back crowds and help Iraqis escape the flames.

Dead and injured lay on the pavement. Others -- dazed from the blast -- shuffled silently and aimlessly down the street, blood streaming from their faces.

The U.S. military command said about 20 people were killed and more than 60 wounded, including three U.S. civilians and three American soldiers. The military initially reported two of the dead were Americans working for the Pentagon but later said their nationalities were unknown and they were not Defense Department employees.

No group claimed responsibility, but anti-U.S. insurgents that the military links to Saddam supporters have targeted both Americans and Iraqis who cooperate with the U.S.-led coalition in previous bombings.

The death toll surpassed that of a homicide bombing in Khaldiyah, west of Baghdad, which killed 17 people on Dec. 14, the day after Saddam's arrest. It was nearly as deadly as the 2,200-pound car bombing blast at U.N. headquarters in Baghdad on Aug. 19, which killed 22 people including top U.N. envoy Sergio Vieira de Mello.

Brig. Gen. Mark Hertling, deputy commander of the 1st Armored Division, said the blast was caused by a homicide bomber. "It certainly was a vehicle-borne bomb, suicide bomb. There was evidently someone in the car," Hertling said in a televised interview.

Col. Ralph Baker, commander of the 2nd Brigade of the 1st Armored Division, said the white Toyota pickup carried the explosives and discounted witness accounts that more than one vehicle was involved.

U.S. soldiers scurried for cover as they saw the vehicle approach the entrance, witnesses said. The bomb-laden truck exploded within 50 to 100 feet from the checkpoint.

"My friend was standing behind me in the line when the explosion happened," said Nabil Abdul Zahar. "There were lots of injured. I called for help, and no one came to help me. He died right there on the ground."

Iraqi police announced on loudspeakers that coalition forces would pay $2,500 to anyone providing information on the perpetrators.

Also Sunday, the military reported that an explosive device being transported in a car exploded Saturday near a U.S. Army patrol in the central city of Tikrit, Saddam's hometown, killing two Iraqis in the vehicle including a relative of Saddam. There were no U.S. casualties, and the military did not draw a connection between it and the truck bomb in Baghdad.

Baker said he was unable to say what type of explosive was employed but that "typically we see PE4 as the principal explosive that they use." PE4 is a military-grade plastic explosive difficult to obtain from civilian sources.

He said the fact that the bomber was unable to penetrate the security cordon "speaks highly of the level of security that we maintain" but that "anamolies like car bombs" are likely to continue.

U.S. and Iraqi authorities were quick to point out that the victims of the Baghdad bombing were mostly Iraqi civilians rather than American occupation troops.

"Once again, it is innocent Iraqis who have been murdered by these terrorists in a senseless act of violence," Bremer said in a statement. "Our determination to work for a stable and democratic future for this country is undiminished."

The U.S.-appointed Iraqi Governing Council blamed the "heinous crime" on terrorists allied with Saddam. "This is yet another stigma on the foreheads of the mass grave regime and its terrorist allies inside and outside [the country], who have no value for sacred things or human lives," the council said in a statement.

The attack occurred one day before Bremer, Annan and U.S.-appointed Iraqi officials are to meet at the United Nations in New York to discuss possible solutions to a political deadlock over a U.S. plan to hand over power to a provisional Iraqi government by June 30.

The Bush administration, which had shunned U.N. involvement in Iraq, has signaled it is now anxious for the world body to play a role in as Washington prepares to hand over sovereignty to a new Iraqi leadership by June 30.

Annan withdrew all international U.N. staff from Iraq after two bombings at U.N. headquarters and a spate of attacks on humanitarian targets. Annan's concern has been whether the situation was secure enough for the world body to return.

The bombing may have been a signal to the world organization to stay out of Iraq and a warning to Iraqis against cooperating with occupation forces. The blast occurred at the main gate used by Iraqis to enter the sprawling palace compound.

The coalition headquarters is one of the most heavily protected areas in Baghdad. U.S. soldiers guarding the gate usually stand about 20 yards from the road behind coils of barbed wire and concrete barriers.

Last fall, the Green Zone saw several mortar and rocket attacks, including the Oct. 26 rocket barrage against the Al-Rasheed Hotel in which a U.S. lieutenant colonel was killed and 18 people were wounded

Deputy Defense Secretary Paul Wolfowitz, one of the architects of the Iraq war, was in the hotel at the time but escaped injury. The last large explosions in the center of Baghdad occurred Monday when mortars exploded near the river.