An underwater magnitude 8.7 earthquake has struck in the Indian ocean off the coast of Indonesia prompting a tsunami warning.
Aftershocks reported after the first quake were reportedly even stronger than the initial quake, said the Reuters news agency, and prompted fresh tsunami alerts across the region.
Seismologists from the US Geological Survey (USGS) said the first earthquake hit near Aceh province on the island of Sumatra.
IN PICTURES:Indonesia Reacts To Tsunami Warning
The Pacific Tsunami Warning Center (PTWC) said a tsunami had been generated, but its strength was not yet known. Reuters quoted one official who said the height of the tsunami was around 17cm.
"Earthquakes of this size have the potential to generate a widespread destructive tsunami that can affect coastlines across the entire Indian Ocean basin," the Pacific Tsunami Warning Center said.
A later warning said that it could be several hours before the threat could be assumed to have passed.
People in Aceh province and the east coast of India rushed to higher ground.
Indonesian President Susilo Bambang Yudhoyono said the country would "remain vigilant".
"Our warning system is working well," he said. "I have ordered the national relief team to fly immediately to Aceh to ensure the situation is under control."
Prime Minister David Cameron is currently in Indonesia on a tour of Asia.
At a press conference where he announced a deal to sell Airbus aircraft to Indonesia, Cameron said that Britain would be ready to help the country "at this time of worry":
The BBC said that in Jakarta buildings shook for up to five minutes during the quake.
The tremors were said to have been felt as far afield as the eastern Indian coast and Thailand.
The quake was located 33km under the sea, the USGS said. Its epicentre was about 495km from Banda Aceh, the captial of Aceh province.
View Indonesia Earthquake in a larger map
Initially the quake was reported as magnitude 8.9 but that was later downgraded.
Indonesia is located on the so-called 'ring of fire', a deadly region of major seismic activity and one of the most active faultlines.
In 2004 an earthquake off the coast of Sumatra resulted in a tsunami which killed around 230,000 people on the coast of the Indian ocean.
That quake was one of the longest ever recorded and was reported to be between magnitude 9.1 and 9.3.