A powerful 8.6 magnitude earthquake and strong aftershocks struck off Indonesia on Wednesday, sending people as far away as southern India scurrying from buildings and raising fears of a disastrous tsunami - a repeat of 2004.

Indonesian President Susilo Bambang Yudhoyono said there were no immediate reports of casualties or damage in Aceh, the Indonesian province closest to the initial earthquake. But an aftershock of almost the same magnitude, less deep that the first quake, hit soon after he finished speaking.

The tremors also sent tens of thousands scurrying out of high-rise buildings and houses in several Indian cities from Guwahati and Kolkata to Chennai and Kochi.

The first quake off the Indonesian coast at 2.08 p.m. swayed buildings in Aceh province in Indonesia. A while later, the second temblor struck.

'Moved Earth horizontally'

Bruce Presgrave of the US Geological Survey told BBC that Wednesday's quake moved the earth horizontally, rather than vertically, and so did not displace large volumes of water.

In India, experts quickly ruled out a tsunami threat but fishermen were warned against venturing into the sea. In Chennai, 16 ships were hurriedly asked to leave the port. All cargo handling was stopped.

A high alert was announced at the Kudankulam Nuclear Power Project in Tamil Nadu's Thirunvelveli district, about 650 km from Chennai. Many parts of Tamil Nadu, Kerala and Karnataka were shaken.

In New Delhi, National Disaster Management Authority vice chairman V. Shashidhar Reddy said: "Initially there was a tsunami threat but it is not so now. The alert was issued as a precaution."

Kolkata's Metro operations were shut shortly after the tremors. Sources claimed a few buildings in the city had developed cracks.

'OH MY GOD'

Men and women rushed out of offices as fear gripped the city, the panic captured on CCTV cameras. Many scampered down several flights of stairs to get on to the roads. Some buildings on Park Street developed cracks.

The story was same in Chennai and Bengaluru.

"Oh, my God! The tremors lasted 30-35 seconds, and everyone came running out," said Vishnu Ram of Chennai. "The tsunami alert is scary."

For a long time, people remained on the streets of cities, till panic subsided.

Chennai's Marina beach - one of the world's longest - was quickly emptied of people by police.

In Kerala, legislator K.V. Abdul Khader said he was asleep on the fifth floor of the legislator's hostel when the ground shook. Like thousands of others, he too fled to safety.

Andhra Pradesh Chief Minister N. Kiran Kumar Reddy directed all nine coastal districts to be on high alert and take help from the army and navy if needed.

Ring of fire

Panic was also reported from Patna and other places.

The home ministry asked authorities in Andaman and Nicobar Islands and all the east coast states to issue a coastal alert.

"They have been asked to advise fishermen not to venture out in the sea," the ministry said.

A 9.1 magnitude earthquake struck off Sumatra in 2004, triggering a tsunami that killed over 200,000 people in 14 countries.

In India, thousands were killed, the maximum in Tamil Nadu.

Indonesia is on the Ring of Fire, an arc of fault lines circling the Pacific Basin prone to frequent earthquakes and volcanic eruptions.

Wednesday's earthquake came a little over a year after a magnitude 9 temblor struck off Japan caused a tsunami. Over 15,000 people were killed in the March 2011 disaster.

The tsunami warning covered Indonesia, India, Sri Lanka, Australia, Myanmar, Thailand, the Maldives, Bangladesh, Britain, Malaysia, Mauritius, Syechelles, Pakistan, Somalia, Oman, Madagascar, Iran, the United Arab Emirates, Yemen, Tanzania, Mozambique, Kenya, South Africa and Singapore.