Specialist police officers cordoned off a part of Croydon while probing a "suspicious vehicle" on Tuesday afternoon.
Nearby buildings were also evacuated in the western end of the town centre, near to a bus station.
And trains were stopped from calling at West Croydon station, although lines have since reopened through the station.
One local said the area had been "completely paralysed" by the ongoing incident.
Another had posted: "Should be interesting getting home from Croydon tonight, suspicious car found, whole place on lockdown..."
The Met said on Twitter that "specialist officers" attended 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.
"Road closures are in place. Motorists should use alternative routes. Bus & train services have also been disrupted."
Transport for London advised on its website that there is no service between Reeves Corner and East Croydon.
Disruption lasted until around 6pm.
Croydon has a population of more than 190,000 people.
