Pub-goers were forced to leave The Drapers Arms in Stevenage when a large crowd descended on the venue.
The crowd allegedly caused a "disturbance" at the pub in High Street at around 3.30pm on Sunday, May 8.
When staff refused to serve them, the fire alarm was pulled and police called.
A Hertfordshire Constabulary spokesperson said: "It was reported that a large number of people had entered the premises, and when staff refused to serve them the fire alarm was pulled.
"Officers attended and moved the group on. The pub was then closed for the rest of the day.
"Information about the disturbance was shared to a number of other local pubs in the High Street for awareness so they could plan accordingly."
They added that it was up to pubs whether they wanted to shut down for the day.
"The decision to close is down to the individual premises and not the police," the spokesperson said.
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