The United Nations has said the devastating blast at its Baghdad headquarters has dealt a blow to its operations in Iraq but will not stop its work in the country.
Such an attack on the UN is thought to be unprecedented
"Such terrorist incidents cannot break the will of the international community to further intensify its efforts to help the people of Iraq," the UN Security Council said in a statement after holding an emergency session.
The blast killed at least 17 people, including UN top envoy Sergio Vieira de Mello, and wounded many others.
UN Secretary General Kofi Annan - who cut short his holidays in Finland - spoke of his "deepest regret" over the death of his envoy and said everyone at the UN was shocked and dismayed at the "murderous" attack.
Earlier, UN chief spokesman Fred Eckhard, at the UN headquarters in New York, said the attack and the death of Mr Vieira de Mello "will have a serious impact on our work in Iraq".
"It is a tragedy, I think, not only personally but also a setback politically for the UN mission," he said.
Pakistan's ambassador to the UN, however, suggested the organisation might have done more for Iraq.
"We need to make a greater effort to convince the Iraqi people that the United Nations is acting for their welfare and that we are a presence that is designed to help them come back to normalcy," said Munir Akram.
The blast triggered indignation worldwide but also expressions of determination to press on with the UN's work.
US President George W Bush said that "the civilised world will not be intimidated and these killers will not determine the future of Iraq".
He added: "The terrorists who struck today have again shown their contempt for the innocent. They showed their fear of progress and their hatred of peace.
"They are the enemies of the Iraqi people. They are the enemies of every nation that seeks to help the Iraqi people."
Mexico's UN Ambassador, Adolfo Aguilar Zinser, called on the Security Council to "show its strength" in a display of unity before an "abominable act of terrorism".
UK Foreign Secretary Jack Straw said the "outrageous, totally unjustified" attack had been carried out by "very ruthless people of the type who sustained the Saddam regime".
He said that the resolve of the coalition and the UN Security Council would not be affected.
Russia condemned the "barbaric act", which had, it said, been aimed at "undermining the already difficult process of post-war stabilisation in Iraq".
UN workers watched in horror as the tragedy unfolded
German Chancellor Gerhard Schroeder said the "criminal attack" was "clearly carried out by forces that do not want the rebuilding of Iraq to take place in peace and freedom".
"The death of... Sergio Vieira de Mello represents a particularly painful loss," Mr Schroeder said in a telegram sent to Kofi Annan, offering Germany's "full support".
Brazilian President Luiz Inacio Lula da Silva declared three days of official mourning for Mr Vieira de Mello - who was born in Rio de Janeiro.
He said Brazil's top international diplomat had been "a victim of the insanity of terrorism".
Syria's ambassador to the UN, Fayssal Mekdad, who is also the current president of the Security Council, said the UN should intensify its work in Iraq, rather than curtail it as a result of the attack.
Flags at the UN building in New York have been lowered as a mark of mourning for Mr Vieira de Mello and the other victims of the attack.
Many UN workers gathered in corridors and around television sets visibly shocked by the loss of the man whom Mr Eckhard called "a rising star".
The 55-year-old Brazilian diplomat's career of more than 30 years with the UN had included work in Bosnia, Africa and East Timor before his post in Iraq.
He was highly regarded and talked about as a likely future UN secretary general.
The BBC's Jane Standley says colleagues described Mr Vieira de Mello as a compassionate man, equally at home with presidents as with ordinary people who needed his help.
Mexico's Mr Zinser said: "The spirit, the vision, the optimism, and the energy of Sergio will remain in Iraq."
Kofi Annan described the loss of his top envoy as a crime not only against the UN but against Iraq itself.
Commonwealth Secretary General Don McKinnon said Mr Vieira de Mello's death was "a huge loss for those committed to peace in the world".
Mr Vieira de Mello was the most senior UN diplomat to be killed in a terror attack since the 1940s.