Emotional ceremonies began across Asia on Thursday to remember the 220,000 people who died two decades ago when a tsunami devastated coastal areas around the Indian Ocean, in one of the worst natural disasters in human history.

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 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.

Indonesia suffered the highest death toll, with more than 160,000 people killed along its western coast.

In Thailand, where half of the more than 5,000 dead were foreign tourists, unofficial vigils were expected to accompany a government memorial ceremony. (AFP)