Rescue worker Songwut Wangpon, speaking at the scene of a huge pile of rubble that was once a high-rise building under construction, told reporters earlier that seven people had been found alive.
Thailand’s National Institute for Emergency Medicine earlier said 43 people were trapped in the rubble – but the country’s deputy prime minister Phumtham Wechachai has now increased that number to 81.
Footage from Bangkok, around 560 miles (900km) from Mandalay, shows the under-construction building in the Chatuchak area crashing to the ground as people run away from the scene.
Follow live: Myanmar earthquake latest
The reports came from Thailand after eyewitnesses said three people died while praying when a mosque partially collapsed in the Bago Region of Myanmar.
Meanwhile local media has reported that at least two people died and 20 were injured after a hotel collapsed in Aung Ben.
Images have also emerged of a destroyed temple in the Myanmar capital Naypyidaw.
They are some of the first details to emerge from Myanmar about the natural disaster – which comes as the country is in the grips of a civil war.
The earthquake took place at a shallow depth of six miles at around 12.50pm local time (6.20am UK time).
A 6.4 magnitude aftershock was felt around 12 minutes later near Mandalay, the United States Geological Survey (USGS) said.
Myanmar’s ruling military junta said a state of emergency has been declared in Sagaing Region, Mandalay Region, Magway Region and northeastern Shan State, Nay Pyi Taw Council Area, and Bago Region.
The junta added in a statement: “The government has ordered a rapid investigation of the damage in these areas, We will carry out relief and relief operations promptly. We will also work to provide necessary disaster relief and humanitarian assistance.
“The Disaster Management Committee will continue to release timely information as it becomes available. We urge the public to be cautious about possible rumours and cooperate with the national government and local authorities in disaster relief and relief efforts.”
The Red Cross humanitarian agency has said downed power lines are adding to challenges for their teams trying to reach Mandalay and Sagaing regions and southern Shan state.
It comes as Myanmar’s state media outlet, the Global New Light, reports that several buildings have collapsed in five cities and towns in the country after the earthquake.
The outlet also reports that two bridges collapsed and an expressway was damaged.
It did not give details of any casualties.
An official from the United Nations Office on Humanitarian Affairs has said initial reports suggest the earthquake has caused significant damage in the centre of Myanmar.
Meanwhile, hundreds of people poured out of buildings in Bangkok after the tremors, with many buildings evacuated.
The city is home to 17 million people and many live in high-rise apartments.
Alarms went off in buildings as the earthquake hit around 1.30pm local time (6.30am UK time).
Footage on social media has shown water being thrown down the side of buildings from rooftop swimming pools
Residents were evacuated down staircases of buildings and hotels in the densely-populated centre of the city.
They remained in the streets, seeking shade from the midday sun in the minutes after the quake.
All flights in and out of Bangkok are operating normally following the earthquake, the country’s civil aviation department said on Friday.
Thailand’s deputy prime minister has said no state of emergency has been declared in the city.
