A man's body has been found at the Pegasus Theatre in Oxford.
Police are the scene of the venue on Magdalen Road.
The public are not currently being allowed in the building.
Officers say the man's death is unexplained but is not believed to be suspicious.
Staff have told the Mail that they have been warned by police they are not allowed to leave the building until further notice.
Gill Jaggers, spokeswoman for the theatre, said she did not know how long staff would have to stay inside.
She said: “We’re all sitting in the office working like normal.
“We are right at the back of the building, so we are not involved in the activity at all.
“The police have told us to stay put until they’ve finished what they’re doing.
“I believe the police are very professional and are able to deal with it.”
No further details have been revealed at this stage.