Patients and staff are being evacuated from the site, which is opposite the Houses of Parliament, and no-one is being allowed inside, according to eyewitnesses.
Westminster Bridge and Lambeth Palace Road are closed, according to BBC Radio London Travel.
All staff, wards, departments and operating theatres inside the hospital have been locked down, according to an NBC producer.
Joe Sheffer wrote: “Incident at St Thomas’ hospital appears to be in south wing according to staff inside hospital.
“All staff, wards, departments and operating theatres have been locked down inside St Thomas’ hospital. Rumors abound on internal whatsapp groups, but nobody seems clear what is going on.”
One witness said: “Something happening at St Thomas’ Hospital. My partner who works in cardiac has been evacuated upstairs, anyone know what going on? Nobody allowed in and out of the hospital.”
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