A mushroom cloud of dust and debris swept through the streets of northern Bangkok on Friday as panicked residents ran for their lives after an under-construction skyscraper came crashing down following a powerful earthquake.

The construction site of a new 30-storey government building quickly turned into a disaster scene, with people jumping into cars to escape or shrieking as they fled on foot.

Workers in hard hats and orange hi-vis jackets were engulfed by dust as the concrete stack fell, with dozens who couldn't get away trapped under the rubble.

Rescuers at the collapse site were dwarfed by an enormous mound of rubble and tangled metal struts, just metres from the bustling Chatuchak Market which is hugely popular with tourists.

Some of the concrete facade and metal bars at the bottom of the tower survived the collapse, AFP photographs showed.

Deputy Prime Minister Phumtham Wechayachai told reporters that at least three workers had been killed, with 81 more trapped inside when the building came down.

'Such a devastating impact'

The 7.7-magnitude quake violently shook buildings across Bangkok -- where strong tremors are almost unheard of -- leaving workers and shoppers rushing into the street in shock.

"At first, I thought I was sick -- like I was getting dizzy or about to faint. Then I noticed the lanterns were moving," said Hongsinunt, who like many other office workers fled her Bangkok building.

Dramatic video footage showed the tremor rocking a high-rise hotel, with water from its rooftop pool whipping over the building's edge.

"I was shopping inside a mall when I noticed some signs moving, so I quickly ran outside," said Attapong Sukyimnoi, a broker.

"I knew I had to get to an open space -- it was instinct."

Terrifying tremors also rocked the northern city of Chiang Mai, popular with tourists.

Receptionist Baitoey Pradit Sa On said when the quake hit all the guests rushed from her hotel.

"It was chaotic... even the water splashed out of the pool," she said, pointing at the soaked area around it.

Strong tremors were also felt in neighbouring China and Myanmar, where roads were pulled up and a major hospital was declared a "mass casualty area".

Bangkok rescuers were focused on the skyscraper collapse site, racing there to find survivors.

"I heard people calling for help, saying 'help me'," Worapat Sukthai, deputy police chief of Bang Sue district, told AFP.

"We estimate that hundreds of people are injured but we are still determining the number of casualties."

"I fear many lives have been lost. We have never experienced an earthquake with such a devastating impact before."