Hundreds of people were evacuated London's Old Vic Theatre during a performance on Saturday.
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The Independent cited police as saying they were responding to an unspecified security threat. The theater ultimately tweeted that "The incident was found not to be suspicious."
Theatergoers tweeted that a matinee performance of "Woyzeck" starring John Boyega was halted by theater staff and audience and cast were told to leave.
Photos showed police tape cordoning off the area around the theater, near the city's busy Waterloo Station, which handles both trains and the tube. Police said officers were at the theater but did not giving further details.
"We have been evacuated as a precaution," tweeted the theater as the situation developed. "Audience safety is our priority. We are liasing with the Met police."
That tweet was followed with an "all-clear" 20 minutes later: "The incident was found not to be suspicious. Thank you for your understanding."
Britain has been under heightened security after Monday's deadly bomb attack at an Ariana Grande concert in Manchester.