According to a press statement from the venue, the audience of Brendan Cowell's Happy New was "over half full" at the time, with several initially believing the destruction was part of the performance.
The man, who is described as being in his 20s, smashed up the set, threatened audience members with violence and then ran out of the theatre.
The statement continues: "Obviously, this was a very frightening experience for audience, cast and stage management alike. Fortunately, the pub is well covered with CCTV, so producers believe that there is a strong chance of catching the person responsible ... The police were also called."
One theory is that the perpetrator was among a group of football fans who regularly come to the pub to watch games on the big screen.
Robert Shaw, the producer and director of Happy New, commented: "I am appalled that my company and audience members were subjected to this malicious, wholly unprovoked attack on artists doing their job and earning their living. I am determined that this cowardly act of assault and criminal damage will not stop artists from earning their living or audiences from coming to watch our very popular production."
Despite the unforeseen interruption, last night's performance continued through to its conclusion. Happy New, which stars Helen Duff, Ed Birch and Harry Potter's Alfred Enoch, runs until 23 March.