Specialist police officers blew up a "suspicious vehicle" in Croydon on Tuesday.
Nearby buildings were evacuated in the western end of the town centre, near to a bus station, as officers swooped.
Trains were stopped from calling at West Croydon station, and motorists told to find alternative routes.
The Met initially said on Twitter that "specialist officers" were at the scene in south London.
"Cordons are in place near #WestCroydon BR Station after officers became aware of a suspicious vehicle at around 15:00hrs," it said.
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"Specialist officers are attending to assess. Some buildings/ shops evacuated as a precaution."
A later statement added: "Police conducted a controlled explosion of the vehicle as a precaution."
One local said the area had been "completely paralysed" by the incident.
Disruption was expected until at least 6pm.
Croydon has a population of more than 190,000 people.
