The epicentre was just over 60 kilometres (40 miles) north-west of the Sikkim state capital Gangtok, where at least 60 people were injured and the town 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," he added. Tremors were felt in Nepal, Bhutan, Bangladesh and the Indian cities of Guwahati and Kolkata, as well as 1,000 kilometres away in 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, and the death toll may rise as reports arrive from remote communities.
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 been blocked by landslides," Gyatso said.
The Press Trust of India news agency said that police rescued 15 foreign tourists in the north of Sikkim, a popular destination for trekkers, but it did not give their nationalities. Nine other people died in India, including one reportedly killed in a stampede by panicked residents in Bihar state and four who were buried when a house fell down near Darjeeling. As many as 1,000 prisoners at Jalpaiguri prison in West Bengal state were escorted from their cells under guard after the building was damaged by the quake, officials reported.
Manish Sharma, a doctor attending a conference in Gangtok, told the NDTV news channel that guests in his hotel had all run for the doors as soon as the first tremors were felt.
"I am standing in front of the legislative assembly and I can see one of the outer buildings... the upper side is in two parts," Sharma said. "I can see light coming out of that particular portion. It has not collapsed but it is in two parts. One part has moved aside," he said. "The police are trying to calm people down."
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.