London Bridge was closed off at about 1.20pm
Bomb squad experts were seen arriving at the scene as Londoners and tourists were quickly moved to the side.
The bridge has been closed to drivers and both the train and bus stations have been evacuated.
Witnesses reported lots of police and fire services at the scene, with more police arriving.
A Met Police spokesman said: “We were called to a security alert in London Bridge street at 1.40pm. The incident is ongoing.”
Disruption to Southeastern services are expected until 4pm, the beleaguered train company said.
Network Rail tweeted: "The London Bridge Bus Station has been evacuated."
Southeastern Railway tweeted: "Because of a security alert at London Bridge, the station has had to be evacuated and at present no trains can pass through. More to follow."
This is a breaking story.
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