Trains have been diverted and will not stop at the London Bridge station while emergency services deal with an incident, the train company Southern said on Twitter.
People have been urged to stay away from the vicinity of the station.
According to the police, officers were called to London Bridge station at 12.33 pm following reports of a suspicious item on board a train.
"Officers were called to London Bridge station at 12.33pm following reports of a suspicious item on board a train," British Transport Police (BTP) said.
British police said that the station has been closed as a precaution while specialist officers prepare an assessment.