The Curious Incident of the Dog in the Night-Time has won the Olivier Award for Best New Play:
Watch highlights from the Olivier Awards, hosted by Hugh Bonneville and Sheridan Smith, on ITV at 10:15pm tonight.
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.