Sergio Vieira de Mello, a 55-year-old veteran Brazilian diplomat who was nearing the end of his four-month mission, was in his office when the explosion ripped through the building about 4:30 p.m. and was trapped in the rubble.
President Bush, at his ranch in Crawford, Texas, condemned Tuesday's truck bombing, calling the attackers "enemies of the civilized world."
"These killers will not determine the future of Iraq," Mr. Bush said. "Every sign of progress in Iraq adds to the desperation of the terrorists and the remnants of Saddam's brutal regime."
According to witnesses, a cement truck exploded at a concrete wall outside the Canal Hotel, where the U.N. was based, but there were conflicting reports about whether the truck was parked or trying to drive through the security barrier.
Video of the blast scene showed several people with injuries ranging from lacerations to severe wounds. CBS News Correspondent Kimberly Dozier said preliminary reports indicated the bomb may have blown away half the targeted building.
The building was not considered an especially weak target, Dozier said, because it had U.S. and U.N. guards and a wide perimeter. At the time of the blast, there was a press conference underway at the hotel for a U.N. program on clearing land mines in Iraq.
U.S. Black Hawk helicopters could be seen flying toward the scene. Many people came to the bombing site to look for relatives who worked there, reports CBS News Correspondent Randall Pinkston.
Bernard Kerik, the former New York City police commissioner who is rebuilding the Iraqi police force, told reporters that evidence suggested the attack was a suicide bombing.
Asked if al Qaeda was behind the attack, Kerik said, "It's much too early to say that. We don't have that kind of evidence yet."
Vieira de Mello reluctantly took leave from his post as the U.N. commissioner for human rights to take the Iraq assignment, the toughest in the U.N., at Secretary-General Kofi Annan's request. He began work June 2 and would have finished his assignment at the end of September, though the U.N. spokesman in Baghdad, Salim Lone, said many U.N. officials wanted him to stay on.
Annan said Vieira de Mello was "an outstanding servant of humanity."
"The loss of Sergio Vieira de Mello is a bitter blow for the United Nations and for me personally," Annan said in a statement. "I can think of no one we could less afford to spare."
A senior UNICEF official also was seriously wounded in the blast, U.N. officials said.
A delegation from the U.S. Congress had just arrived in Baghdad and was getting a briefing on security in the country when the blast occurred, Sen. Kay Bailey Hutchison, R-Texas, told CBS News. None of the U.S. lawmakers was hurt.
A senior U.S. official, speaking on condition of anonymity, said in Baghdad that the truck did not breach the security wall that was erected around the hotel within the past month. He said it was parked on an access road just outside the compound. Witnesses said it was uncertain if the truck was parked or trying to break through the barrier.
The official estimated the amount of explosives was double that used in the attack on the Jordanian embassy almost two weeks ago in which 19 people were killed.
The embassy attack was thought to be the first such terrorist-style bombing in the Iraqi capital since Saddam Hussein's fall.
L. Paul Bremer, the top U.S. civilian administrator in Iraq, walked through the scene of destruction as workers dug through the rubble with their hands trying to find people. There was a 15-yard wide hole in the ground.
Bremer had tears in his eyes and hugged Hassan al-Salame, an adviser to Vieira de Mello. A part of the building collapsed near him. People cried: "Watch out. Watch Out."
"We will leave no stone unturned to find the perpetrators of this attack," he said.
The U.N. Security Council, which was briefed about the bombing at a closed-door meeting, called the blast a "terrorist attack." U.S. diplomats were pushing for the council to adopt a statement condemning the bombing.
Several countries denounced the attack. Russia's Foreign Ministry called the explosion a "barbaric act" and said it was "aimed at undermining the already difficult process of postwar stabilization in Iraq," the Interfax news agency reported.
In other developments in Iraq: