From Baghdad itself to UN Headquarters in New York, from Geneva to Buenos Aires and in other far-flung outposts of its worldwide operations, staff observed a minute's silence, top officials and local representatives paid homage and relatives mourned the 22 people who perished in the blast of 19 August 2003.
In Baghdad, almost within site of the ruins of the Canal Hotel where top UN envoy Sergio Vieira de Mello and many of his colleagues died, dozens of women and men, both Iraqi and international UN staff members, gathered in the Diwan School in the International Zone.
Secretary-General Kofi Annan's Special Representative Ashraf Qazi delivered a message from Mr. Annan and paid tribute to the victims, survivors, those injured and their families for their determination to press on with their mission in serving the Iraqis and other peoples around the world.
The attackers "will not deter us from going ahead with the UN," the Officer-in-Charge of the UN Administration in Baghdad, Anas Darwash, said in re-pledging the commitment of Iraqi UN staff to pursue their work with dedication in helping the country to emerge from successive wars and build a secure future. A religious service including an imam and two priests was held.
At the UN's European headquarters in Geneva, Mr. Annan himself led the commemoration ceremonies, pledging to do all in his power to reinforce security in the face of the "new and more intimidating" form of danger that faces the world body as it goes about its global peacekeeping and humanitarian operations.
At the UN's world Headquarters in New York Deputy Secretary-General Louise Fréchette presided over ceremonies attended by representatives of the bereaved families as well as colleagues who were injured in the bombing.
A documentary film celebrating the achievements of the fallen was shown, and the UN flag that had flown over the Canal Hotel that fateful day one year ago was unveiled in a place of honour in the Trusteeship Council Chamber, damaged by the blast but still proud.
The Security Council issued a press statement mourning the "irreparable loss" in the attack on people whose only mission was to help Iraq "build a better future of peace, justice, sovereignty and full independence," and it strongly condemned the continuing terrorist attacks on representatives of the international community.
Hundreds of staff staged a silent march around the fountain in front of the UN complex carrying placards with messages such as "We will never forget you," "Remember the fallen" and "Never again."
At three ceremonies joined by video-link in New York, Geneva and Amman - the Jordanian capital where many international staff relocated after the attack - UN flags on the stage were dipped in salute and a minute of silence was observed after Mr. Annan's address.
Members of the bereaved families or their representatives were led to the podium where they lit candles for each of the fallen, their images projected on a screen, and identical commemorative plaques were unveiled.
Similar ceremonies were held in many of the world's current hotspots where the UN is on the front line with peacekeeping or humanitarian missions in the field.
In Kabul, a tree was planted and a plaque mounted at the UN operation centre. The head of the UN Assistance Mission in Afghanistan (UNAMA), Jean Arnault, said the magnitude of what happened a year ago served as a reminder of the scale of the risk that the world body faces in many of its operations today.
At the Monrovia headquarters of the UN Mission in Liberia (UNMIL), a country torn apart last year by civil war before the UN arrived to support the peace process, Mr. Annan's Special Representative Jacques Paul Klein embraced four staff members who survived the Baghdad tragedy and are currently serving in the West African nation.
In Sudan and Burundi, two countries most recently in the headlines because of massacres and brutalities, UN missions held similar memorials, as they did in former hotspots that have enjoyed greater stability more recently, as in Nicosia, Cyprus, where more than 100 UN military, police and civilian personnel were in attendance.
On the front line of natural catastrophes, too, in Dhaka, capital of Bangladesh, where disastrous floods recently killed 600 people, left 20 million more in need of food aid, damaged or destroyed three million homes and caused losses estimated at $7 billion, UN Development Programme (UNDP) Administrator Mark Malloch Brown paid tribute to the fallen and re-dedicated the world body to its principles of aiding all humanity.
And so it was in Nairobi, Kenya, the UN's African headquarters, and in countless other field offices of a world body whose missions gird the planet, from tiny Timor-Leste to the vast expanses of the Democratic Republic of Congo (DRC), from the Caucasus mountains of Georgia to the tropical shores of Haiti.
WATCH VIDEO Duration: 41 minutes