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Officials in Burlington, Vermont are raising concerns about the volume of tickets issued by Donald Trump's campaign for a rally Thursday night, saying that overcrowding could jeopardize public safety.

Trump's campaign has reportedly distributed about 20,000 free tickets for an event the Flynn Center that seats about 1,400.

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"If Phish was holding a free concert at the Flynn and gave away 20,000 free tickets, we would cancel the event out of public safety concerns,” Burlington Police Chief Brandon del Pozo told the Burlington Free Press. "We are committed to accommodating the campaign because political speech is the very essence of the First Amendment."

On Wednesday night, Trump tweeted that attendees should arrive early for the rally and that admittance is "first come, first serve."

Huge crowd expected tomorrow night! VT Police say first come, first serve. Arrive early! — Donald J. Trump (@realDonaldTrump) January 6, 2016

He added in a message Thursday morning:

Massive crowd in VT tonight. Venue not big enough. Officials say NO to outside event and sound system. Arrive early! — Donald J. Trump (@realDonaldTrump) January 7, 2016

The Trump campaign's effort to turn out huge lines of supporters outside the event is no subtle signal to Democratic presidential candidate Bernie Sanders.

Burlington is a stronghold of support for the Vermont senator, who formerly served as the mayor of the city.

Sanders' campaign headquarters is located directly across the public square from the venue where Trump's rally is scheduled to be held at 7pm ET.