BANGKOK — A 7.7 magnitude earthquake rocked Thailand and neighboring Myanmar midday on Friday, causing the collapse of a high-rise building under construction in Bangkok and forcing hundreds of thousands to evacuate from their homes and workplaces.

The midday temblor was followed by a strong 6.4 magnitude aftershock.

Possible casualties from the building collapse near Bangkok’s popular Chatuchak market are not yet known, police said, and there was no immediate information on how many workers were on the site at the time.

A dramatic video circulated on social media showed the multi-story building with a crane on top collapsing into a cloud of dust, while onlookers screamed and ran.

People in Bangkok evacuated from their buildings were cautioned to stay outside in case there were more aftershocks.

The U.S. Geological Survey and Germany’s GFZ center for geosciences said the earthquake was a shallow 10 kilometers (6.2 miles), with an epicenter in Myanmar, according to preliminary reports.

“All of a sudden the whole building began to move. Immediately there was screaming and a lot of panic,” said Fraser Morton, a tourist from Scotland, who was in one of Bangkok’s many malls shopping for camera equipment.

“I just started walking calmly at first but then the building started really moving, yeah, a lot of screaming, a lot of panic, people running the wrong way down the escalators, lots of banging and crashing inside the mall.”

Rescuers work at the site of a high-rise building under construction that collapsed after a 7.7 magnitude earthquake in Bangkok, Thailand on Friday.

Sakchai Lalit/AP

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Sakchai Lalit/AP

Like Morton, thousands of people poured into Benjasiri Park from nearby shopping malls, high rises and apartment buildings along Bangkok’s busy Sukhumvit Road.

Many were on phones trying to reach loved ones as others sought shade from the hot early afternoon sun. Others stared up fearfully at the tall buildings in the densely packed part of the city.

“I got outside and then looked up at the building and the whole building was moving, dust and debris, it was pretty intense,” Morton said. “Lots of chaos.”

The sound of sirens echoed throughout central Bangkok and vehicles filled the street, leaving some of the city’s already congested streets gridlocked. The elevated rapid transit system and subway were shut down.

City hall declared the city a disaster area to facilitate interagency aid and emergency help.

Paul Vincent, a tourist visiting from England, was at a streetside bar when the quake struck.

“The next thing, everybody came on the street, so there was a lot of screaming and panicking, which obviously made it a lot worse,” he said.

As he came onto the street himself, he said he saw a high-rise building swaying and water was falling from a rooftop pool.

“When I saw the building, oh my God, that’s when … it hit me,” he said. “There was people crying in the streets and, you know, the panic was horrendous really.”

In Mandalay, Myanmar’s second-largest city and close to the epicenter, the earthquake damaged part of the former royal palace and buildings, according to videos and photos released on Facebook social media.

Damaged pagodas are seen after an earthquake on Friday.

Aung Shine Oo/AP

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Aung Shine Oo/AP

While the area is prone to earthquakes, it is generally sparsely populated, and most houses are low-rise structures.

In the Sagaing region just southwest of Mandalay, a 90-year-old bridge collapsed, and some sections of the highway connecting Mandalay and Myanmar’s largest city, Yangon, were also damaged.

Residents in Yangon rushed out of their homes when the quake struck. There were no immediate reports of injuries or deaths.

In the capital Naypyitaw, the quake damaged religious shrines, sending parts toppling to the ground, and some homes.

To the northeast, the earthquake was felt in Yunnan and Sichuan provinces in China and caused damage to houses and injuries in the city of Ruili on the border with Myanmar, according to Chinese media reports.

Videos that one outlet said it had received from a person in Ruili showed building debris littering a street and a person being wheeled in a stretcher toward an ambulance.

The shaking in Mangshi, a Chinese city about 100 kilometers (60 miles) northeast of Ruili, was so strong that people couldn’t stand, one resident told The Paper, an online media outlet.

A resident of Kunming, the provincial capital of Yunnan, told The Paper that her ceiling lamp was swinging wildly and the shaking lasted more than 10 seconds.

In Bangkok, alarms went off in buildings as the earthquake hit around 1:30 p.m., and startled residents were evacuated down staircases of high-rise condominiums and hotels.

The greater Bangkok area is home to more than 17 million people, many of whom live in high-rise apartments.

Rescuers carry an injured from the site of a high-rise building under construction that collapsed after a 7.7 magnitude earthquake in Bangkok, Thailand on Friday.

Sakchai Lalit/AP

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Water from high-rise rooftop pools sloshed over the side as they shook, and debris fell from many buildings as the long-lasting earthquake rattled the city.

“I have experienced earthquakes twice before in Myanmar, but that was only one second, one big bang, but here it went on for at least, I’d say, a minute,” said Zsuzsanna Vari-Kovacs, a Hungarian resident of Bangkok, who had just finished eating at a restaurant when the quake hit.

“My husband was in a high-rise, I think that’s even worse.”

Thailand’s Department of Disaster Prevention said the quake was felt in almost all regions of the country.

Prime Minister Paetongtarn Shinawatra called an emergency meeting to assess the impact of the quake.