The 7.7-magnitude tremor hit northwest of the city of Sagaing on Friday afternoon at a shallow depth, the United States Geological Survey said. A 6.4-magnitude aftershock hit the same area minutes later.
In the Thai capital, a 30-storey building under construction collapsed, trapping 43 workers, police and medics said.
The massive building intended for government offices was reduced to a tangle of rubble and twisted metal in seconds, footage shared on social media showed.
"When I arrived to inspect the site, 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," he said.
Across the border in Myanmar, a team of AFP journalists were at the National Museum in Naypyidaw when the earthquake struck.
Pieces fell from the ceiling as the building began shaking. Uniformed staff ran outside, some trembling and tearful, others grabbing cellphones to try to contact loved ones.
Roads nearby were buckled and broken by the tremors and the route to one of the city's biggest hospitals was jammed with traffic.
The hospital was a "mass casualty area" after the quake, officials said.
An ambulance made its way between vehicles, a paramedic shouting "cars, move aside so the ambulance can get through."
At the 1,000-bed hospital, the wounded were being treated in the street outside, intravenous drips hanging from their gurneys.
Some writhed in pain, others lay still as relatives sought to comfort them.
The tremors send people into the streets across both countries.
"I heard it and I was sleeping in the house, I ran as far as I could in my pyjamas out of the building," Duangjai, a resident of the popular northern Thailand tourist city Chiang Mai, told AFP.
Sai, a 76-year-old Chiang Mai resident, was working at a minimart when the shop started the shake.
"I quickly rushed out of the shop along with other customers," he said.
"This is the strongest tremor I've experienced in my life."
- Buildings damaged -
The quake forced the suspension of some metro and light rail services in Bangkok, where Thai Prime Minister Paetongtarn Shinawatra declared a state of emergency.
Earlier she said she had interrupted an official visit to the southern island of Phuket to hold an "urgent meeting" after the quake, according to a post on X.
Tremors were also felt in China's southwest Yunnan province, according to Beijing's quake agency, which said the jolt measured 7.9 in magnitude.
Earthquakes are relatively common in Myanmar, where six strong quakes of 7.0 magnitude or more struck between 1930 and 1956 near the Sagaing Fault, which runs north to south through the centre of the country, according to the USGS.
A powerful 6.8-magnitude earthquake in the ancient capital Bagan in central Myanmar killed three people in 2016, also toppling spires and crumbling temple walls at the tourist destination.
The breakneck pace of development in Myanmar's cities, combined with crumbling infrastructure and poor urban planning, has also made the country's most populous areas vulnerable to earthquakes and other disasters, experts say.
The impoverished Southeast Asian nation has a strained medical system, especially in its rural states.
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