The earth began to move 40km below the seabed, a massive rupturing of the earth's crust off the northwestern tip of Sumatra.
A section of seabed 1,000km long rose up to 30m at a spot approximately 250km southeast of the city of Banda Aceh and 1600km northwest of the Indonesian capital of Jakarta.
Millions of people were living, fishing and holidaying around the Bay of Bengal and on the coast of Thailand and Malaysia, hundreds of kilometers from the epicenter. They were not to know that the gentle shaking that caused sky-scrapers in Singapore and Chiang Mai in northern Thailand to sway would unleash a devastating tsunami, bringing a wall of water crashing down on their shores.
Mohammed Firdus, 36, a telephone operator from Bireuen, Aceh province, was sitting on the porch of his house, about 200m from the sea, when the earthquake struck.
Then he heard a rumbling, but this time the ground was not shaking. Someone came running fast from the beach, shouting, "huge wave, huge wave."
Firdus said: "And then I saw the water. It was a wall at least a meter high coming down the track towards us all. We all immediately turned and ran towards the main road with the water following us."
Officials said it was impossible to say how many people were killed by the earthquake itself because it was followed so quickly by tsunamis striking Aceh province and smaller islands like the popular surfing resort of Nias, where an entire hotel, the Wismata Indah, was washed out to sea.
"The wall of water which came ashore was between 5m and 10m high in many places," said Ari Meridal, a provincial government official in Banda Aceh. "It swept almost everything away for hundreds of meters inland."
Severed communications meant that estimates of casualty figures were imprecise.
"We have heard very little from west Aceh, which is the nearest point to the epicenter," said Raifa Sistani, of the Indonesian Red Cross. "This is a major concern to us because logic says this area should have suffered the most."
The packed Thai tourist resorts on Phuket and Phi Phi islands were the next to be hit with a succession of tsunamis 10m high.
Montri Charnvichai, a resident of Phuket, was on the beach at 10am when the sea water suddenly disappeared and the beach dried.
He said: "Then the first wave hit. It must have been traveling at about 70km per hour, it was very fast. It swept up the beach, carrying everything with it. There were many, many people in the sea at this time, and many of them were tourists. I have no idea what happened to them."
Then the second wave hit, about two minutes after the first. It was 3m high, and crashed into the buildings lining the shore.
Simon Clark, a British photographer holidaying on Koh Ngai, described a huge wave crashing on to the beach, destroying everything in its wake.
"People who were snorkeling were dragged along the coral and washed up on the beach, and people who were sunbathing got washed into the sea," he said.
The tsunami struck Phuket just after 10am, when Christmas revelers were just starting to surface.
"It was like a really, really bad dream," said Dawn Taylor from Stockport, England, who was on Kamala beach. "It was a glorious day and a group of us were enjoying the beach when suddenly we saw this wall of water coming towards us. We just ran. The scale of the devastation is just enormous."