The 8.9 magnitude quake struck under the sea near Aceh in north Indonesia, generating a wall of water that sped across thousands of kilometres of sea.
More than 4,100 died in Indonesia, 4,300 in Sri Lanka and 2,900 in India.
Casualty figures are rising over a wide area, including resorts in Sri Lanka and Thailand packed with holidaymakers.
Sri Lanka: 4,300 dead
Indonesia: 4,185 dead
India: 2,900 dead
Thailand: 310 dead
Malaysia: 28 dead
Maldives: 10 dead
Bangladesh: 2 dead
Source: Government officials
Eyewitness: Tsunami escape
In pictures: Quake disaster
Exact numbers of people killed, injured or missing in the countries hit, are impossible to confirm.
Hundreds are still thought to be missing from coastal regions and, in Sri Lanka alone, officials say more than a million people have been forced from their homes.
Sri Lankan President Chandrika Kumaratunga declared a national disaster and the military has been deployed to help rescue efforts.
Click here for map of affected area
Hundreds of fishermen are missing off India's southern coast, and there are reports of scores of bodies being washed up on beaches.
In Indonesia, communications remain difficult, particularly to the strife-torn region of Aceh where the main quake, early on Sunday morning, was followed by nine aftershocks. Reports speak of bodies being recovered from trees.
Click below to see how the disaster unfolded
A national disaster has also been announced in the low-lying Maldives islands, more than 2,500km (1,500 miles) from the quake's epicentre, after they were hit by severe flooding.
The Indian-owned Andaman and Nicobar islands, much nearer the epicentre, were also badly hit.
Casualty reports could not be officially confirmed, but a police chief told reporters 300 people had died and another 700 were feared dead.
Waves forced out from the earthquake are even reported to have reached Somalia, on the east coast of Africa.
And as far away as the Seychelles, nine people were reported missing as a two-metre surge struck.
Resort 'wiped out'
International aid agencies have called for a rapid response to the emergency to avert further deaths.
The European Union immediately pledged 3m euros (£2.1m) to disaster relief efforts.
The Thai resort of Phuket feels the force of the surge
Messages of condolences have poured in from around the world.
US President George W Bush offered aid to affected nations and expressed sorrow for the "terrible loss of life and suffering".
Harrowing reports of people caught in the devastation and dramatic tales of escape are emerging.
Jayanti Lakshmi, 70, had gone shopping with her daughter-in-law in Cuddalore, southern India. Ms Lakshmi returned to find her son and twin grandsons dead in their hut.
"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," she said.
All of us fear the final death toll, and in particular are worried that many tourists who went out on boat trips this morning have not returned
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In Thailand, hundreds of holiday bungalows are reported to have been destroyed on the popular Phi Phi island.
Resort owner Chan Marongtaechar told AP: "I am afraid there will be a high figure of foreigners missing in the sea, and also my staff."
Indonesia's location - along the Pacific geological "Ring of Fire" - makes it prone to volcanic eruptions and earthquakes.
Sunday's tremor - the fifth strongest since 1900 - had a particularly widespread effect because it seems to have taken place just below the surface of the ocean, analysts say.
Bruce Presgrave of the US Geological service told the Reuters news agency: "These big earthquakes, when they occur in shallow water... basically slosh the ocean floor... and it's as if you're rocking water in the bathtub and that wave can travel throughout the ocean."
Experts say tsunamis generated by earthquakes can travel at up to 500km/h.
my heart goes out to these people. i am sorry to see it. may what ever god they beleive in bless their soul. this is a sad time in that region.