The accident occurred at Promphan intersection on Prachasongkroh Road on Sunday evening.
The bus conductor told INN radio news station that the driver had a seizure while the bus was moving and fell unconscious. The No.12 bus, licence plate 14-6484 Bangkok, was on a run from Huai Khwang to Pak Khlong Talat, near Memorial Bridge.
A motorcycle taxi driver was struck and lost a leg in the crash while two others sustained minor injuries. Names and other details regarding the bus, conductor and the injured are not yet available.
Pol Lt Sahachai Montha, an investigator at Din Daeng police station, said three motorcycles and a pickup truck were damaged in the crash.
As of 7pm, police had yet to question the driver and file any charge against him. Pol Lt Sahachai said police would find out first if he had a seizure before the crash before pressing charges.