Story highlights Trump faced repeated disruptions at his event

The father of a slain Muslim American soldier held up a pocket Constitution at the DNC

Portland, Maine (CNN) A group of protesters at Donald Trump's rally here on Thursday stood up silently during the Republican nominee's speech and held up pocket copies of the Constitution.

They were quickly ejected by campaign staffers and drew fierce boos and jeers from the roughly 1,800 Trump supporters packed into the auditorium, the capacity of the venue.

Protesters at @realDonaldTrump's event in Portland, Maine stand & hold up copies of the Constitution as Trump speaks pic.twitter.com/xgsEIa7AuK — Jeremy Diamond (@JDiamond1) August 4, 2016

The protest was a reference to Khizr Khan's high-profile condemnation of Trump at the Democratic National Convention last week. Khan is the father of a Muslim-American soldier who was killed in action in the Iraq War.

The seven protesters held up pocket copies of the Constitution issued by the American Civil Liberties Union. It was not immediately clear whether the demonstrators were affiliated with a particular organization.

The ACLU responded on Twitter, saying: "Glad to see people are standing up for constitutional principles using their ACLU pocket Constitutions!"

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