Shoppers fled some of the Thai capital's many malls in panic and some high-rise office blocks were evacuated after they swayed.
People in Hanoi, the Vietnamese capital, also felt the tremor and office workers were ordered to leave some tall buildings.
"The building shook quite a bit and we were told to evacuate," said Michael DiGregorio, a programme officer at the Ford Foundation, a United States philanthropic organisation with offices on the 15th floor of a Hanoi building.
The United States Geological Survey (USGS) said on its website the quake hit at 08h56 GMT with its epicentre 148km from the ancient Lao capital of Luang Prabang.
People in Luang Prabang, a world heritage site on account of its centuries-old Buddhist pagodas and temples, said they felt only minor shaking.
Longitude and latitude coordinates given by the USGS placed the epicentre in the sparsely populated, mountainous northern province of Luang Namtha, in the heart of the "Golden Triangle" opium-producing region near the Burma and Chinese borders.
The tremors caused some panic in the neighbouring province of Oudomxai, but no damage or casualties, Lao Foreign Ministry spokesperson Yong Chanhthalansy said.
He had no word from Luang Namtha, a remote province mainly populated by hill tribes living in wooden or bamboo huts. -- Reuters