Armed police guarded exits at Southend Hospital while staff treated a criminal from Chelmsford Prison.
Officers were on standby outside the observation ward on Wednesday morning and others waited in a vehicle near the main entrance.
A police spokeswoman said a prisoner had been taken to the hospital needing treatment.
She was unable to reveal his identity but confirmed that because of the nature of his crime he had an armed police escort.
Kate Burgess, director of the observation ward, said: "We often have patients transferred from prisons but it is quite unusual for them to have armed guards."
The prisoner was expected to be returned to Chelmsford in the afternoon.
Picture, top: Watchful - an armed policeman stands guard outside Southend Hospital as a prison patient is treated inside.
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