In Thailand, Buddhist monks chant a prayer for those who died in the tsunami.

In Indonesia's Aceh Province, officials sound a new tsunami-warning siren as part of the memorial ceremonies for the more than 169,000 who died there.

In the Sri Lankan town of Galle, people pause as a clock tower counts down the moments before 9:26 a.m., the exact time a year ago the tsunami struck the country.

Mourners gathered also in India, Malaysia and even in Sweden, which lost more than 500 citizens in the tsunami. They were among the thousands of Europeans who were visiting the region's warm beaches for the Christmas holidays when the tsunami struck. More than 2,500 tourists from around the world were among those lost.

The massive earthquake and the tsunami it triggered on December 26 last year swept across the Indian Ocean, and wiped out more than a quarter million lives. Entire communities were swept away. In what was in many ways a global tragedy, hundreds-of-thousands of people were left homeless, jobless, without schools or hospitals.

And around the globe, people rose to help the devastated communities. More than $13.5 billion were raised to rebuild the region. Volunteer aid workers and military missions from many countries took up the task of bringing food, water, medicine and shelter to the victims.

Leaders from around the world joined in the memorial observances Monday. In a recorded message to the people of Aceh, Kofi Annan, the secretary-general of the United Nations, said that, despite the record outpouring of aid and a massive rebuilding effort, the suffering continues for many.

"Yet in some ways, the most challenging days lie ahead," he said. "Bread-winners desperately need to regain secure livelihoods. Hundreds-of-thousands of families need to re-establish themselves in permanent homes, and communities need to rebuild."

In his video message to Aceh, President Bush praised the strength and resilience of the survivors who are rebuilding their lives and communities.

"One year after the tsunami, Americans of all faiths join nations around the world in the spirit of unity, remembrance and resolve. May Almighty God comfort all those affected by the tsunami, and give them strength," said Mr. Bush.

Among the many tasks that remain for those involved in the reconstruction is finding ways to prevent another such tragedy. Indian Ocean nations are trying to develop a regional system to warn of possible tsunamis. And Mr. Annan says that the United Nations is working to learn from the tsunami, to find ways to speed aid around the world the next time disaster strikes.