Two major railway stations were closed and 1,450 people were evacuated after a gas main ruptured in central London.
Heaven nightclub was cleared and guests at a nearby hotel were moved into emergency accommodation after the alarm was raised in the Strand at around 2am on Tuesday.
Charing Cross and Waterloo East stations were shut, with services diverted into Victoria, Cannon Street and Blackfriars.
A Network Rail spokesman said: “We will reopen these stations as soon as we have clearance from emergency services that it is safe to do so.
“We are sorry for the inconvenience caused.”
Charing Cross is London’s fifth busiest station, used by about 42 million passengers a year.
The Strand, a major route through central London, was closed and a 165-yard (150m) cordon put in place while National Grid engineers tried to isolate the leak.
Westminster City Council said a respite centre was opened at the Queen Mother Sports Centre in Victoria.
“More than 1,000 people from the Heaven night club and 350 residents and business personnel were evacuated,” a spokesman said.
High levels of natural gas were detected at Craven Street, near Charing Cross station, London Fire Brigade said.
A spokesman added: “We are assisting police on the Strand after a ruptured gas main was discovered using detection equipment and high levels of natural gas were detected in the atmosphere.
“As a precaution, approximately 1,450 people have been evacuated. They are from a hotel and a night club.
“The Strand is completely closed and we are asking people to avoid the area.
“We do not know the cause of the gas leak at this stage. It is ongoing and engineers are down at the scene trying to isolate the leak.
“There are still high readings of natural gas in the area. We are working to sort it out as quickly as possible.”
Witnesses at Heaven nightclub in Villiers Street described “chaos” as people tried to leave “as fast as possible”.