An earthquake measuring 8.7 on the Richter scale has struck the Indian Ocean off the Indonesian Island of Sumatra.
The quake occurred 1400km northwest of the Indonesian capital Jakarta. The epicentre was located about 200km off the mainland.
The shock hit in the middle of the night, lasting for 2 to 3 minutes, and was felt as far away as Bangkok to the north and the Maldive Islands to the west.
As the US Geological Survey advised that coastal areas within a 1000km of the epicentre be evacuated, thousands of people across the region made for higher ground.
Dozens of people are believed to have been killed by the quake on the outlying Indonesian island of Nias. Hundreds more are missing.
Police said at least 70% buildings in the town of Gunungstoli on Nias Island had collapsed.
Scenes of panic have been reported across the region, as have power blackouts.
Tremors from the earthquake were felt in Malaysia, sending residents of hotels and high-rise buildings fleeing in the capital Kuala Lumpur.
Thailand issued a tsunami warning for provinces along its Andaman Sea coast. Similar warnings were issued by authorities in Sri Lanka and India.
Fears of a tsunami have now receded.
Experts say the earthquake is an aftershock of the massive quake which struck the region on 26 December leaving 273,000 people dead or missing.
