Survivors launched a boat laden with flowers, candles and incense in the first ceremony Saturday to mark one year since the Indian Ocean tsunami swept away at least 216,000 lives in one of the world's worst natural disasters in memory.

Peter Pruchniewitz, 68, who was swept from his hotel room and lost a friend to the waves, returned from Zurich, Switzerland to attend the ceremonies. Asked why, he said simply, "to remember."

The commemoration in Thailand was the first of hundreds to be held on the grim anniversary in the dozen countries hit by the earthquake-spawned waves last Dec. 26.

Amid the mourning, survivors and officials were taking stock of the massive relief operation and peace processes in Sri Lanka and Indonesia's Aceh province, the two places hardest hit by the tsunami. In both cases, success has been mixed.

At Bang Niang beach in Thailand's Phang Nga province, mourners including Western tourists who were caught in the disaster placed offerings into a brightly colored, bird-shaped boat that was floated into the Andaman Sea as members of the Moken, or sea gypsy, tribe chanted and pounded drums.

The Moken believe the ceremony helps ward off evil spirits.

A private memorial service for British citizens and two candlelight ceremonies were planned for later Saturday on the nearby island of Phuket.

In hardest-hit Indonesia, workers on Saturday scaled the minarets of the imposing 16th century mosque in the provincial capital of Banda Aceh, replacing missing tiles and slapping on a fresh coat of whitewash in preparation for special services on Monday.

Thousands of survivors have been rehoused in Aceh, but agencies say they are only about 20 percent of the total number needing new homes and the landscape is still one of devastation in many places.

But the tsunami did bring one positive side effect in Aceh — it resulted in a cease-fire between the government and guerillas that ended a decades-old separatist conflict.

No such progress was made in Sri Lanka, where disputes over aid delivery and an upsurge in violence blamed on separatist Tamil Tiger rebels have dashed hopes that the tsunami would end the country's long-running civil conflict.

On Saturday, troops patrolled the streets of the capital, Colombo, amid boosted security for tsunami ceremonies.

Exactly one year ago Monday, the most powerful earthquake in four decades — magnitude 9 — ripped apart the ocean floor off Sumatra island, displacing millions of tons of water and sending giant waves crashing into Indian Ocean coastlines from Malaysia to east Africa.

A dozen countries were hit by surging walls of water powerful enough to level buildings and sweep small ships miles inland. Entire villages in Indonesia and Sri Lanka were swept away, five star resorts in Thailand were swamped, and in the Maldives whole islets temporarily disappeared.

At least 216,000 people were killed or disappeared in the waves, according to an assessment by The Associated Press of government and credible relief agency figures for each country hit — though the United Nations puts the number at least 223,000.

The true toll will probably never be known — many bodies were lost at sea and in some cases the populations of places struck were not accurately recorded.

Almost 400,000 houses were reduced to rubble and more than 2 million people left homeless, the U.N. says. The livelihoods of 1.5 million were swept away.

The world responded with donor pledges of some $13.6 billion. Rebuilding has started in some places, and fishing boats and seeds have been handed out to kick-start ruined village economies.

But many refugee camps are still full and residents rely on aid handouts to survive. Concerns linger about the pace of rebuilding.

Former President Clinton, the U.N. special envoy for tsunami recovery, said much work remained to be done and the international community faces a "critical challenge" in following through on its promises of help.

"One year ago ... millions of ordinary people across the globe rallied to the immediate aid of communities devastated by the tsunami," Clinton said in remarks prepared for the anniversary and published Saturday in the International Herald Tribune.

"Now our collective challenge is to finish the job, to leave behind safer, more peaceful and stronger communities."