On December 26, 2004, a 9.1-magnitude earthquake off Indonesia's western tip generated a series of massive waves that pummelled the coastline of 14 countries from Indonesia to Somalia.
In Indonesia's Aceh Province where more than 100,000 people were killed, a siren rang out for three minutes at the Baiturrahman Grand Mosque, followed by Islamic prayers to kick off a series of memorials scheduled around the region.
Indonesian mourners will attend a mass grave ceremony and a communal evening prayer in provincial capital Banda Aceh, while beachside memorials and religious ceremonies were scheduled in Sri Lanka, India and Thailand, some of the worst hit countries.
"I thought it was doomsday," said Hasnawati, a 54-year-old teacher who goes by one name, at the mosque which was damaged by the tsunami.
"On a Sunday morning where our family were all laughing together, suddenly a disaster struck and everything's gone. I can't describe it with words."
Victims of waves as high as 30 metres (98 feet) included many foreign tourists celebrating Christmas on the region's sun-kissed beaches, bringing the tragedy into homes around the globe.
The seabed being ripped open pushed waves at double the speed of a bullet train, crossing the Indian Ocean within hours.
A total of 226,408 people died as a result of the tsunami, according to EM-DAT, a recognised global disaster database.
There was no warning of the impending tsunami, giving little time for evacuation, despite the hours-long gaps between the waves striking different continents.
But today a sophisticated network of monitoring stations has cut down warning times.
Candlelight, train vigils
Indonesia suffered the highest death toll, with more than 160,000 people killed along its western coast.
"I hope we will never experience that ever again," said Nilawati, a 60-year-old Indonesian housewife who lost her son and mother in the tragedy and was attending Thursday's ceremonies.
"I learned the devastation of losing a child, a grief I can't explain with words. It feels like it just happened yesterday. Whenever I am reminded of it, it feels like all the blood rushes out of my body."
The disaster also ended a decades-long separatist conflict in Aceh, with a peace deal between rebels and Jakarta struck less than a year later.
In Sri Lanka, where more than 35,000 people perished, survivors and relatives were to gather to remember around 1,000 victims who died when waves derailed a passenger train.
The mourners will board the restored Ocean Queen Express and head to Peraliya -- the exact spot where it was ripped from the tracks, around 90 kilometres (56 miles) south of Colombo.
A brief religious ceremony will be held with relatives of the dead there while Buddhist, Hindu, Christian and Muslim ceremonies are also organised to commemorate victims across the South Asian island nation.
In Thailand, where half of the more than 5,000 dead were foreign tourists, unofficial vigils were expected to accompany a government memorial ceremony.
At a hotel in Phang Nga province, there will host a tsunami exhibition, a documentary screening and introductions by government and UN bodies on disaster preparedness and resilience measures.
Nearly 300 people were killed as far away as Somalia, as well as more than 100 in the Maldives and dozens in Malaysia and Myanmar.
"My children, wife, father, mother, all of my siblings were swept away," said Indonesian survivor and fisherman Baharuddin Zainun, 70.
"The same tragedy was felt by others as well. We feel the same feelings."
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