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An 8.9 magnitude earthquake under the sea near Aceh, north Indonesia, at 0759 local time (0059 GMT) generated the biggest tsunami the world has seen for at least 40 years.
The wall of water fanned out across the Indian Ocean at high speed and slammed into coastal areas with little or no warning.
Officials in Indonesia, Sri Lanka and India have all reported death tolls in the thousands and the figures are expected to rise sharply over the next few days.
Indonesia is thought to be the worst-hit country in the region, with 4,185 confirmed dead.
In Sri Lanka, over 3,500 people have died and one million more have been made homeless. President Chandrika Kumaratunga has declared a national emergency.
Hundreds of fisherman are missing off the southern Indian coast and witnesses have reported scores of bodies being washed up on beaches.
Other countries hit by the tsunami include Malaysia, Thailand, the Maldives, the Seychelles and the Indian-owned Andaman and Nicobar islands.
Harrowing stories from the disaster zone are beginning to emerge.
Jayanti Lakshmi, 70, lost her son and twin grandsons while out shopping with her daughter-in-law in Cuddalore, southern India.
She said: "I wish I had died instead of the others, my daughter-in-law would have a life. I can't bear to watch her pain."
And a father in Sri Lanka watched as his entire family was swept away by the sea.
"It dragged my wife away, then my two-month-old twins," he said. "Then I watched my seven-year-old son drown."
Large part of the affected area are popular holiday destinations and many resorts have been badly hit.
In Thailand, hundreds of holiday bungalows have been destroyed on the popular Phi Phi Island. Tourists from all over the world are thought to be among the dead.
Health experts now fear that many more could die as diseases like typhoid, cholera and malaria spread rapidly.
United Nations Emergency Relief Co-ordinator Jan Egeland said many of the affected areas had dense populations living in sub-standard housing.
"This may be the worst natural disaster in recent history," he added.
The tsunami killed more than 200,000 people in 13 countries. At least 128,000 people died in Indonesia alone.
Many months later, bodies were still being discovered in some countries. In Thailand a British forensic team continues its work identifying up to 2,000 victims.
There was a massive international response. Six months after the disaster about $12bn (£7bn) around the world had been pledged in aid.
But rebuilding in many of the stricken areas is slow and thousands of people remain homeless.
The UN has said it expects much of the reconstruction work to take up to five years.
Governments affected by the disaster are now working to build a tsunami early warning system.
The Boxing Day sea surge was triggered when an earthquake caused the sea floor to jolt vertically by about 10m (33ft), which displaced hundreds of cubic kilometres of water.
The resulting waves travelled at speeds of up to 800km/h (497mph).
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