UN Secretary General Kofi Annan has spoken of his deep sense of loss at the deaths of 22 people killed in the 2003 bombing of the UN's Baghdad HQ.
Mr Annan was leading the main commemoration ceremony in Geneva on the first anniversary of the attack.
He said the bombing showed that "some are determined to block our work at any price", but declared: "Our belief in the cause of peace is undiminished."
After Mr Annan spoke, relatives of the victims lit candles in their memory.
The secretary general then returned to the podium to unveil a plaque in honour of the victims.
Mr Annan described the attack, which led the UN to pull out of Iraq, as "cold-blooded murder" and said he prayed that the perpetrators would not go unpunished.
He told relatives of the dead that his heart went out to them, adding: "I can only imagine the strength and courage you have had to muster to endure the past year."
'Agony'
The UN secretary general said the attack was a "unique blow" to the organisation, which had brought it "face to face with danger in a new and more intimidating form".
He said he had lost "22 wonderful, talented and generous friends" who had been sent to bring stability and peace to Iraq.
"Their mission was cut short when they were brutally and viciously taken away from us," he added.
Mr Annan said he felt "agony, discouragement and a deep sense of loss" at their deaths.
"I do not think anyone could have known the impact those tragic events had on me, except perhaps my wife," he said.
Events to mark the anniversary were also held in Amman, Jordan and New York.
Among the dead was the head of mission, Sergio Vieira de Mello, a Brazilian widely respected for his work in areas including East Timor and Kosovo.
Mr Vieira de Mello's mother Gilda, widow Annie, and adult sons Adrien and Laurent attended the Geneva ceremony.
"I have come to represent my son. He would have told me that I was too old to travel," Gilda Vieira de Mello, who said she had not taken a plane in 17 years, told reporters.
"I want to know from the UN how this tragedy could happen, why there wasn't more security. I want truth and justice for my son. He gave his life," she said.
Security fears
It is still not known for sure who was behind the 2003 attack - which UN staff term "our 11 September", referring to the 2001 attacks in the US.
Jordanian militant Abu Musab al-Zarqawi - blamed by the US for multiple bombings and kidnappings in Iraq - claimed to be behind the blast, but little more is known.
A scathing report on the attack, published earlier this year, led to the sacking of one senior official and the demotion of another. It called security arrangements before the bombing "dysfunctional".
Some relatives have demanded the findings of an FBI investigation into the bombing be released.
Last week, a small, low-profile UN team headed by former Pakistani diplomat Ashraf Jehangir Qazi returned to Iraq, but Mr Annan says it is still too dangerous to restore a full presence.