Officials in the Andaman and Nicobar Islands prepped for evacuation procedures though by 5.30 pm, an all-clear is likely to be formally announced.
The minutes after the quake saw emergency drills in Tamil Nadu and Andhra Pradesh. On the beaches at Chennai, policemen used megaphones to ask people to leave, with little effect. Mobile phone networks were jammed in the city till 4 pm. In Andhra Pradesh, fishermen were asked to return from sea.
In the Andaman and Nicobar Islands, civilians were asked to move to higher ground. In Kolkata, metro trains were stopped at 2.40 pm for nearly an hour; as a precaution, passengers were evacuated.
The quake was felt in Singapore, Thailand and India. High-rise apartments and offices on Malaysia's west coast shook for at least a minute.
In India, tremors were reported in Mumbai, Kolkata, Bangalore and the southern part of Chennai. The tremors lasted for a few seconds. In Bhubaneshwar, people were seen running out of their homes and offices. No damage has been reported so far.
NDTV viewer AG Kalidass emailed from Bangalore to say, "We were feeling our building shaking." Another viewer Shyam Agarwal writes, "I am from Kolkata. Just now a few minutes back heavy stroke of earthquake has been observed here."
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 U.S. Geological Survey said the powerful quake was centered 20 miles (33 kilometers) beneath the ocean floor around 308 miles (495 kilometers) from Aceh's provincial capital.
Indonesia straddles a series of fault lines that makes the vast island nation prone to volcanic and seismic activity.
A giant 9.1-magnitude quake off the country on Dec. 26, 2004, triggered a tsunami in the Indian Ocean that killed 230,000 people in 13 Indian Ocean countries, including Thailand, Sri Lanka and India.