A dramatic video circulated on social media on Friday showed the multi-storey building with a crane on top collapsing into a cloud of dust, while onlookers screamed and ran.
Police told The Associated Press they were responding to the scene near Bangkok's popular Chatuchak Market, and had no immediate information on how many workers were on the site at the time of the collapse.
The midday temblor was followed by a strong 6.4 magnitude aftershock, and people in Bangkok fled from their buildings were cautioned to stay outside in case there were more.
"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."
Like thousands of others in downtown Bangkok, Morton sought refuge in Benjasiri Park - away from the tall buildings all around.
"I got outside and then looked up at the building and the whole building was moving, dust and debris, it was pretty intense," he said.
"Lots of chaos."
The US Geological Survey and Germany's GFZ centre for geosciences said the earthquake was a shallow 10km, with an epicentre in Myanmar, according to preliminary reports.
In Mandalay, the country'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.
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.
In Bangkok, alarms went off in buildings as the earthquake hit about 1.30pm, and startled residents were fled down staircases of high-rise apartment buildings and hotels.
The greater Bangkok area is home to more than 17 million people, many of whom live in high-rise apartments.
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.
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.
