The US Geological Survey said the quake struck the small, landlocked Himalayan state of Sikkim -- which borders Nepal, Bhutan and Tibet -- at around 6:10pm (1240 GMT).
The epicentre was just over 60 kilometres (40 miles) northwest of the Sikkim state capital Gangtok, which was plunged into darkness by a power cut after the quake.
"There is no electricity. Everybody is out on the road," Gangtok resident C.K. Dahal told the CNN-IBN television news channel.
"We all ran out our houses, some even jumped out of their windows. You can see some buildings that have developed cracks," Dahal added.
Powerful tremors were felt across a wide region, including Nepal, Bhutan, Bangladesh and the Indian cities of Guwahati and Kolkata, and even as far away as the Indian capital New Delhi.
In Nepal, police said three people were killed, including a motorcyclist and his eight-year-old daughter, when a wall collapsed at the British Embassy compound in the capital Kathmandu, 270 kilometres west of the epicentre.
"Another two died in a separate incident in eastern Nepal," national police spokesman Binod Singh told AFP.
A budget debate in Nepal`s parliament was stalled for 15 minutes while lawmakers leapt to their feet and fled the chamber as the entire building shook.
Telephone landlines to Sikkim, India`s least populous state, were knocked out and mobile networks were swamped, making communication with the affected area difficult.
The quake was followed by two strong aftershocks, one with a magnitude of 6.1.
Sikkim Chief Secretary Karma Gyatso told AFP that five people had been killed and 60 injured in and around Gangtok as the result of mudslides, building collapses and falling debris.
"We have reports of dozens of collapsed houses, and roads to many towns have ben blocked by landslides," Gyatso said.
Another person was reported killed in a stampede by panicked residents in a town in the eastern state of Bihar.
In New Delhi, Prime Minister Manmohan Singh called an emergency meeting of the National Disaster Management Authority, and Cabinet Secretary Ajit Seth said that air force planes carrying rescue teams and relief supplies had been despatched to Sikkim. (*)
Editor: Kunto Wibisono
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