The quake measured magnitude 6.1 and struck not far from the northern city of Luang Prabang, the US Geological Survey reported.

Buildings swayed in both the Thai and Vietnamese capitals. People reportedly fled shopping centres and buildings were evacuated in Bangkok.

However, there were no reports of damage or injuries.

The earthquake struck 155km (97 miles) west-north-west of Luang Prabang at just before 1600 local time (0900GMT), the USGS reported.

The epicentre is believed to be in a fairly remote area of western Laos.

But it was little felt in Luang Prabang. One resident spoke of feeling "disorientated" for a couple of minutes, while others said they felt little or no shaking.

An official in the Laos capital Vientiane said the quake lasted around 10 seconds, and there were no immediate reports of casualties or damage from the area.

Checking buildings

But there were scenes of panic in the Thai capital, Bangkok, some 800km (500 miles) south of the quake zone.

Many office blocks were swiftly evacuated after the earthquake shook the city, with its occupants gathering in the streets outside.

"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 the AFP news agency.

The quake was also felt strongly in the area of northern Thailand bordering Laos. Some people were evacuated in the city of Chiang Mai.

Officials said they were still checking buildings but had no reports of damage or injuries.

Hundreds of kilometres to the east, in the Vietnamese capital, Hanoi, the quake was felt mostly on the upper floors of high rises.

Around 700 people fled Vincom Tower, one of the tallest buildings in the city, when it started to sway, a company spokeswoman told the Associated Press.

"It was shaking for about three minutes, and I think this was the strongest shaking we have ever felt," Nguyen Thu Lan said.