India withdrew a national tsunami alert issued after huge earthquakes struck off Indonesia on Wednesday, the Indian tsunami warning centre said.
"Thankfully, the danger has passed," a scientist at the centre told Reuters.
Disaster officials had earlier warned waves as high as 3.9 meters could hit parts of Andoman and Nicobar islands.
A few thousand people were evacuated from the more vulnerable islands, a police official said.
Indonesia said it was checking for damage and casualties but remarkably, no such reports had been received for several hours after the quakes, including in Aceh, the closest province and the area decimated by the disaster eight years ago.
However, some areas close to the epicentre are remote so it could take some time to find out if there was any damage.
Many people were frightened of further tremors.
"It's dark out here but I am scared to go home," said Mila, a 41-year-old woman taking refuge in the grand mosque in the town of Banda Aceh, the provincial capital.
"I just want to stay alert because I fear there will be more quakes coming. We are human, it is only natural that we have fear, but I really wish we will all be safe."
Waves of up to one metre (3.3 feet) high were seen near islands off Aceh, but Indonesia cancelled a warning for fresh tsunamis. It said the worst-hit area was the thinly populated island of Simeulue, off Aceh's southern coast.
The first quake struck at 0838 GMT and an 8.2 magnitude aftershock just over two hours later, at 1043 GMT. Two more strong aftershocks hit later.
The Hawaii-based Pacific Tsunami Warning Center also withdrew tsunami warnings for the entire Indian Ocean after keeping them in force for several hours.
"Level readings now indicate that the threat has diminished or is over for most areas," the agency's bulletin said.
Thailand and India also withdrew tsunami warnings.
Thailand, Indonesia, Sri Lanka and India were all badly hit in 2004. At least 230,000 people in 13 Indian Ocean countries were killed in the Boxing Day disaster that year, including 170,000 in and around Aceh alone.
Last year, an earthquake and tsunami off Japan's northeastern coast killed at least 23,000 people and triggered the world's worst nuclear crisis in 25 years after waves battered a nuclear power station.
On Wednesday, people near the coast in six Thai provinces were ordered to move to higher ground. Authorities shut down the international airport in the Thai beach resort province of Phuket.
The quakes were about 300 miles (500 km) southwest of Banda Aceh, on the northern tip of Indonesia's Sumatra island, the US Geological survey said. The first was at a depth of 20.5 miles (33 km).
Indonesia's disaster management agency said power failed in Aceh province and people were gathering on high
ground as sirens warned of the danger.
"The electricity is down, there are traffic jams to access higher ground. Sirens and Koran recitals from mosques are everywhere," said Sutopo, spokesman for the agency.
"The warning system worked," Indonesian President Susilo Bambang Yudhoyono said.
Warning sirens also rang out across the Thai island of Phuket, a tourist hotspot that was one of the worst hit areas in the 2004 tsunami.
"Guests from expensive hotels overlooking Phuket's beaches were evacuated to the hills behind and local people were driving away in cars and on motorcycles. Everyone seemed quite calm, the warning had been issued well in advance," freelance journalist Apichai Thonoy told Reuters by telephone.
Out on the streets
Indonesian television showed people gathering in mosques in Banda Aceh. Many others were on the streets, holding crying children.
In the city of Medan, a hospital evacuated patients, who were wheeled out on beds and in wheelchairs.
Wednesday's quakes were felt as far away as the Thai capital, Bangkok, and in southern India, hundreds of office workers in the city of Bangalore left their buildings while the port of Chennai closed down because of tsunami fears.
The quakes were in roughly in the same area as the 2004 quake, which was at a depth of 18 miles (30 km) along a fault line running under the Indian Ocean, off western Indonesia and up into the Bay of Bengal.
Experts said Wednesday quakes were a "strike-slip" fault, meaning a more horizontal shift of the ground under the sea as opposed to a sudden vertical shift, and less risk of a large displacement of water triggering a tsunami.
"The nature of the sideways rupture and sideways movement is not predisposed to cause a bad tsunami, so almost certainly, the crisis has been avoided," said David Rothery, an expert at the Open University in the UK
The quakes were also felt in Sri Lanka, where office workers in the capital, Colombo, fled their offices.
Mahinda Amaraweera, Sri Lanka's minister for disaster management, called for calm while advising people near the coast to seek safety.
"I urge the people not to panic. We have time if there is a tsunami going to come. So please evacuate if you are in the coastal area and move to safer places," Amaraweera told a private television channel.
In Bangladesh, where two tremors were felt, authorities said there appeared to be no threat of a tsunami.
Australia also said there was no threat of a tsunami there.
Why a major tsunami is unlikely | Timeline: disasters to hit Indonesia since 2004 | Largest earthquakes since 1900 | Earthquakes in India