A spokesman for Belfast City Council told the Belfast Telegraph the gates "always close at 9pm unless an event has been advertised".
Second seed Murray beat Polish player Jerzy Janowicz 6-7 6-4 6-4 6-3 in a thrilling display of sporting excellence to reach his fourth consecutive Grand Slam final.
The Scotsman now faces world number one Novak Djokovic in the Wimbledon men's final on Sunday.
Around 100 people were watching the match on the big screen in the grounds of Belfast City Hall yesterday but they had to leave as the gates were closed and the plug was pulled.
Andreas Alm, a teacher from Sweden, was among those left to make their way home.
"I would have liked to stay on a little longer to watch the game," he said.
"I'm cheering for Murray.
"He was in difficulties but came back strong."
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