From worst-hit Aceh province in Indonesia to the tourist beaches of Thailand and tropical Sri Lanka, thousands of survivors, victims' relatives and officials were to hold a minute's silence at the times the waves hit as part of commemoration ceremonies. Indonesia was first to hold the 60-second observance.
The tsunami first smashed into Aceh, which was closest to the epicenter of the magnitude 9 earthquake that sent 33-ft.-high waves roaring across the Indian Ocean at jetliner speeds.
President Susilo Bambang Yudhoyono — on a stage overlooking tranquil blue waters that belied the fury unleashed one year ago — set off a siren at 8:16 a.m. (0116 GMT) to mark the moment.
Hundreds of survivors, foreign dignitaries and aid workers bowed their heads.
"It was under the same blue sky, exactly one year ago that mother earth unleashed her most destructive power upon us," Yudhoyono said.
"The assault began with a massive earthquake but ... that was only a prelude to the horrific catastrophe to come."
At least 216,000 people were killed or disappeared in the waves, The Associated Press found in an assessment of government and credible relief agency figures in each country hit. The United Nations puts the number at least 223,000, though it says some countries are still updating their figures.
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.
People prayed at mass graves Monday for children swept away by the tsunami, walked on white sand beaches battered by the waves and attended prayers at mosques, temples and churches.
"It is important for me to come here to pray for my family, may they rest in peace," said Darmawati, 39, who lost her husband, two daughters and both parents in the disaster.
"I pray that God will give me strength to raise my only son who survived," she said, breaking down in tears at a mass prayer in the Acehnese village of Kajhu.
The tsunami generated one of the most generous outpourings of foreign aid ever known. Some $13 billion was pledged to relief and recovery efforts, the U.N. says, of which 75 percent has already been secured.
But the pace of relief and reconstruction has been criticized.
Looking across the part of Banda Aceh that was hardest hit, it seems like little has been done, CBS News Correspondent Barry Petersen reports – houses turned to hulks, fields littered with debris stretched for miles.
There are a lot of people still living the way they were pretty much in the days after the tsunami, some 70,000 are still living in tents. They're still waiting for the rebuilding to reach them.
Even in the city of Banda Aceh, where a lot of areas were untouched by the tsunami, the earthquake's ground-ripping upheaval destroyed the city's sewer and water system.
"Every meter, every yard of pipe in the city of Banda Aceh is ultimately going to have to be replaced. This is the mother of all home improvement projects," aid worker Paul Dillon said.
While local economies are recovering, 80 percent of the 1.8 million people displaced by the disaster still live in tents, plywood barracks or the homes of family and friends, according to the aid group Oxfam International.
Margareta Wahlstrom, the U.N. assistant secretary-general for humanitarian affairs, said people have to be patient.
"If you don't do things well they will collapse in a couple of years," she told AP on Sunday. "If you don't take time to do proper planning, and ask people what they want ... then you are going to create new problems along the way."
The Indian Ocean region has been struggling since last year to implement a tsunami warning system like the one that already exists for the Pacific with country's disagreeing over who should issue alerts.
Until a sophisticated network of seismic monitoring, satellite communications and underwater sensors is in place, some coastal communities are mapping out escape routes.
Thousands were to take part in a tsunami drill in the Sumatra city of Padang on Monday.