The 8.9 magnitude earthquake that struck off the Indonesian island of Sumatra early on Sunday was the biggest in 40 years.
It triggered waves that reared up into walls of water as high as 10 meters (30 feet) as they hit coastlines in Indonesia, Sri Lanka, India and Thailand.
Aid agencies rushed staff, equipment and money to the region, warning that bodies rotting in the water were already beginning to threaten the water supply for survivors.
Rescue workers also spoke of bodies still caught up on trees after being flung inland by the waves.
"I just couldn't believe what was happening before my eyes," Boree Carlsson said from a hotel in the Thai resort of Phuket.
"As I was standing there, a car actually floated into the lobby and overturned because the current was so strong," said the 45-year-old Swede.
"I heard an eerie sound that I have never heard before. It was a high pitched sound followed by a deafening roar," said a 55-year-old Indian fishermen who gave his name as Chellappa.
"I told everyone to run for their life."
In Indonesia, an archipelago of 17,000 islands, one official said nearly 4,500 people had died.
The worst affected area was Banda Aceh, the capital of Aceh province, where 3,000 were killed. More than 200 prisoners escaped from a jail when the tsunami knocked down its walls.
