* Indonesia's Disaster Mitigation Agency on Wednesday reported a 6.5 magnitude earthquake on the Richter scale following an 8.7 magnitude temblor off the coast of the country's Aceh province, triggering a tsunami watch for the whole Indian Ocean.
JAKARTA, April 11 (Reuters) - An 8.7 magnitude earthquake struck off the coast of Indonesia on Wednesday, sending residents around the region dashing out of their homes and offices in fear.
The Pacific Tsunami Warning Center said a tsunami watch was in effect for the entire Indian Ocean and individual countries, including Thailand, Indonesia, Sri Lanka and India, issued tsunami warnings.
People near the coast in six Thai provinces were ordered to higher ground.
The quake struck 308 miles (500 km) southwest of the city of Banda Aceh, on the northern tip of Indonesia's Sumatra island, at a depth of 20.5 miles (33 km), the U.S. Geological survey said.
Indonesia's disaster management agency said power was down 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 quake was felt as far away as the Thai capital, Bangkok, and in southern India, residents said.
Hundreds of office workers in the Indian city of Bangalore left their buildings while the Indian port of Chennai closed down because of the danger of a tsunami, the port said.
The quake was in roughly in the same area as a Dec. 26, 2004, quake of 9.1 magnitude, which sent huge tsunami waves crashing into Sumatra, where 170,000 people were killed, and across the Indian Ocean.
In all, the 2004 tsunami killed about 230,000 people in 13 Indian Ocean countries, including Thailand, Sri Lanka and India.
The 2004 quake 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.
One expert told the BBC the Wednesday quake as 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 quake was also felt in Sri Lanka, where office workers in the capital, Colombo, fled their offices, and the southern Thai holiday island of Phuket, both of which were hit hard by the 2004 tsunami.
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.
Largest earthquakes since 1900
Here is a factbox showing the 10 strongest earthquakes recorded since 1900, by order of magnitude as Indonesia issued a tsunami warning after a huge 8.7 magnitude earthquake struck off its westernmost province of Aceh on Wednesday.
May 22, 1960 - Chile - An earthquake of magnitude 9.5 struck Santiago and Concepcion, triggering tidal waves and volcanic eruptions. Some 5,000 people were killed and 2 million made homeless.
March 28, 1964 - Alaska - An earthquake and ensuing tsunami killed 125 people and caused about $310 million in property loss. The magnitude 9. 2 qu ake buffeted a large area of Alaska and parts of western Yukon Territory and British Columbia in Canada.
Dec. 26, 2004 - Indonesia - A magnitude 9.1 quake struck off the coast of Aceh province on the Indonesian island of Sumatra, setting off a tsunami that killed more than 226,000 people in Sri Lanka, Thailand, Indonesia, India and nine other countries.
Nov. 4, 1952 - Russia - An earthquake with a magnitude of 9.0 g enerated a tsunami that reached the Hawaiian islands. No lives were lost.
March 11, 2011 - An 9.0 magnitude quake struck Japan, causing many injuries. The U.S. Geological Survey verified the quake at a depth of 15.1 miles and located it at 81 miles east of Sendai, on the main island of Honshu.
-- The quake was the strongest in Japan on record, and a big tsunami followed, which triggered the world's worst nuclear crisis in the 25 years since Chernobyl. More than 15,000 people died from the combination of the earthquake and tsunami.
-- The Philippines, Taiwan and Indonesia all issued tsunami alerts, reviving memories of the giant tsunami which struck Asia in 2004. The Pacific Tsunami Warning Center issued alerts for countries as far away as Colombia and Peru.
Feb. 27, 2010 - Chile - An 8.8 magnitude quake and subsequent tsunami in Chile killed more than 500 people and caused some $30 billion in damage, wrecking hundreds of thousands of homes and mangling highways and bridges.
Jan. 31, 1906 - Ecuador - An earthquake with a magnitude of 8 .8 s truck off the coast of Ecuador and Colombia, generating a tsunami that killed up to 1,000 people. It was felt all along the coast of Central America and as far north as San Francisco and west to Japan.
April 11, 2012 - A huge 8.7 magnitude quake struck Indonesia's Aceh province some 308 miles southwest of the province's capital Banda Aceh.
-- The quake was felt as far away as Singapore, Thailand and India.
Feb. 4, 1965 - Alaska - An earthquake of magnitude 8.7 g enerated a tsunami reported to be about 35 feet (10.7 metres) high on Shemya Island.
March 28, 2005 - A magnitude 8. 6 quake off Sumatra was estimated to have killed 1,300 people, many on Nias island off Sumatra's west coast.
Reuters/U.S. Geological Survey Earthquake
Web site - http://earthquake.usgs.gov/