A section of Samsen Road in front of Vajira Hospital collapsed on Wednesday morning, forming a large hole about 50 metres deep. No injuries were reported.
It happened around 7.13am, with the sinkhole estimated to be 30 metres wide and 30 metres long.
The road between Vajira and Sanghi intersections was closed.
Outpatient services at the hospital were suspended, and about 3,500 inpatients were evacuated from nearby buildings. An initial report indicated that hospital buildings were not damaged.
Residents of nearby apartments were also told to evacuate.
Two electricity poles and a police vehicle fell into the hole after the collapse.
The area was previously closed due to the construction of the Mass Rapid Transport (MRT) Purple Line (Tao Poon and Rat Burana extension).
Authorities have not yet confirmed the cause of the subsidence. Bangkok Governor Chadchart Sittipunt and a team of officials were inspecting the scene.
Mr Chadchart said later the sinkhole formed above the Vajira Hospital railway station, at the junction between the tunnel and the station. The soil flowed into the tunnel, causing surrounding structures to collapse and a large water pipe to break, he said.
Authorities have cut off water and electricity to control the situation and prevent further risks. Other urgent measures ordered included sealing the tunnel leak, assessing the safety of surrounding buildings, monitoring soil movement and managing traffic in the area.
He said another critical concern was rainfall, which could cause more soil to flow into the openings.
Meetings would be held at noon and 6pm daily to monitor the situation, the BMA said.
Prime Minister Anutin Charnvirakul told reporters after inspecting the scene that repairs to the tunnel and road would take at least a year.
