The incident occurred around 7.13am, with the sinkhole estimated to be 30 metres wide and 30 metres long. It formed immediately above Vajira Hospital train station, reports the Bangkok Post.
Traffic between Vajira and Sanghi intersections was closed for safety reasons, according to the Dusit District Office.
Outpatient services at the state hospital were suspended, and about 3,500 inpatients were evacuated from nearby buildings. An initial report indicated that hospital structures were not damaged.
Residents of nearby apartments were also instructed to evacuate.
Two electricity poles and a police vehicle fell into the hole after the collapse. Officials from the Metropolitan Electricity Authority (MEA) were on-site working.
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, specifically 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. A team was set up to evaluate and respond to rainfall scenarios.
Huge subsidence on Samsen Rd in Bangkok
The state-owned hospital had suspended outpatient services for two days. Inpatient services remained operational. Other hospitals under the Bangkok Metropolitan Administration were available for outpatient cases during this period, he said.
Meetings would be held at noon and 6pm daily to monitor the situation, the BMA said.