More than 800 Tube passengers were led through tunnels to safety after being trapped underground in a stalled train for more than two hours.
Ambulances treated some passengers for dehydration
The Central Line train was stopped after another train in front on the approach to Bank station broke down.
Passengers were led through the tunnel and the defective train to Bank station, a London Underground (LU) spokesperson said.
Some passengers said emergency services took too long to respond.
The incident occurred at about 0850 BST.
London Ambulance Service said it sent five ambulances and treated 12 of the 820 people for dehydration.
London Underground (LU) did not reveal how far back through the tunnel the stalled train was.
The incident forced the cancellation of services from Leytonstone to Marble Arch on Friday morning.
LU said it was investigating why the train broke down. Passengers said they were underground for two-and-a-half hours without water.
Central Line services were affected by the incident
Some told BBC London the driver had told them something had fallen off the defective train in front of them, and hit their train.
Some passengers said the emergency services had taken too long to respond.
One woman said: "There were pregnant people on board, they got the pregnant people out first and some people fainted. It was unbearable."
Severe delays have affected other parts of the Central Line. Power was switched off at Bank station and Monument station, which meant Northern Line and Docklands trains were not stopping.
The power has since been restored.
Investigation under way
Transport for London's (TfL) Howard Collins apologised to passengers, but said staff were sent to meet the train within 15 minutes.
He said TfL would try and do all it could to get passengers off trains as quickly as possible in such circumstances.
A London Underground spokesperson said an investigation was under way: "We are working with Metronet who are responsible for the maintenance of Central line trains to identify and fully understand the cause of this incident."
A stretch of a the Central Line was also shut for 90 minutes on Thursday evening while police searched a tunnel for a man on the tracks.
It reopened after being suspended between Holborn, in central London, and White City in west London, from about 1800 BST.
British Transport Police arrested a man after searching the tunnel between Marble Arch station and Lancaster Gate.