Royal United Hospital in Bath has been placed under “lockdown” after reports of a security threat.
Police have warned people “to avoid the area”, while one woman in the hospital said “armed police” had arrived.
Other witnesses told Somerset Live that no one was being allowed in or out, with patients “stuck on wards”.
A spokesman for Avon and Somerset Police said: “We are currently responding to an incident in the vicinity of the Royal United Hospital in Bath.
“Officers are at the scene but at this moment in time would advise people to avoid the area as a precaution.
“We will provide further updates in due course.
“We’d like to reassure the public that all emergency services have well-rehearsed plans for dealing with incidents.There are no reported injuries and our advice to avoid the area at the moment is precautionary.