The quake hit at 0857 GMT, the US Geological Survey said, in the heart of the Golden Triangle, a rugged and forested region where Thailand, Myanmar and Laos meet in what was once a notorious opium smuggling route.
Officials in those countries as well as neighbouring China said there were no immediate reports of damage or casualties.
People in the Thai capital Bangkok, roughly 880 kilometres (550 miles) from the epicentre, poured onto the street as high-rise buildings rocked.
In the Vietnamese capital Hanoi, hundreds of people fled office towers and hotels when the city's tall buildings started to sway.
"I thought I was suffering a stroke or a heart attack -- then I realised it was an earthquake," said Fred Burke, a managing partner of the law firm Baker McKenzie, who was on the 13th floor of a Hanoi office building.
"The building shook for about a minute. The staff were screaming, and we told them to get away from the windows and take cover under the tables."
The US Geological Survey estimated the tremor was centred at a depth of 38 kilometres.
China's Seismological Monitoring Network, using a different scale, reported the quake at a strength of 6.6 while Thailand's meteorological department said a 4.7 magnitude aftershock hit at about 1005 GMT.
Officials in the four Thai provinces near the Laos border -- Chiang Mai, Chiang Rai, Nan and Lampang -- felt the quake, which hit about 60 kilometres northeast of the Thai border, but reported no serious damage or injuries.
"It was the most severe quake ever to hit northern Thailand," said Smith Thammasaroj, director of the National Disaster Centre. He cautioned people to be aware of possible landslides triggered by the earthquake.
Laos reported little immediate impact from the quake, which struck a mountainous region near the Chinese and Myanmar borders, a sparsely-populated traditional opium-growing area dominated by ethnic minorities.
"As far as we know now, there are no victims," Lao foreign ministry spokesman Yong Chanthalangsy told AFP by telephone from Vientiane.
"There are no tall buildings and people live in a very scattered way, in small houses," he said. "The damage shouldn't be very serious."
Residents of the remote town of Oudon Xay felt the ground move for about 10 seconds and fled their buildings, but there were no reports of material damage, he said.
In Luang Nampha, close to the Myanmar border, "people also felt pretty strong jolts," he said. "There was some panic but no property damage and no victims either. The quake lasted about 15 seconds."
The quake was not felt in the Lao capital Vientiane or the former royal capital Luang Prabang, a UNESCO world heritage site, said Yong.
Authorities in the Chinese city of Jinghong, just over the border from the epicentre, said the quake was felt there but there were no reports of injuries or damage.
In the Thai province of Chiang Rai, the top of an ancient pagoda which was under restoration crumbled to the ground, a local official said, while a handful of buildings suffered minor cracks in the cities of Chiang Rai and Chiang Mai.
In Chiang Mai, a city popular with tourists, people evacuated shaking buildings and flooded onto the streets.
"My building shook twice, once very heavily, and the second time was like having a headache -- the floors were sliding," said Viparwan Chaiprakorb, who works on the third floor of an office building in the centre of Chiang Mai.
"People were scared," she told AFP by phone.
Shocked workers huddled on the streets of Bangkok, which rarely feels the effects of earthquakes.
"I have not seen a strong earthquake like this before, my head felt like it was spinning," Nattaya Limngern, a 40-year-old office worker, told AFP.
