The Knight First Amendment Institute at Columbia University in New York said that while the great unwashed can block who they like, a US president is not allowed to block those who disagree with him under the First Amendment of the US Constitution.

Trump's @realDonaldTrump account recently blocked a number of accounts that replied to his tweets with commentary that criticized, mocked or disagreed with his actions.

The Knight First Amendment Institute said in its letter that the blocking represented the suppression of free speech in a public forum.

Alex Abdo, the institute's senior staff attorney, likened Twitter to a modern form of town hall meeting or public comment periods for government agency proposals, both venues where U.S. law requires even-handed treatment of speech.

Eric Goldman, a Santa Clara University law professor who focuses on internet law, said that previous cases involving politicians blocking users on Facebook supported the Knight Institute's position.

Trump could claim his @realDonaldTrump account is for personal use and separate from his official duties as president, Goldman said that defense was "laughable."

Trump also has a presidential @POTUS Twitter account. The Knight Institute said its arguments would apply with "equal force" to both accounts.

Songwriter Holly O'Reilly was blocked on May 28 after posting a brief animated clip of Pope Francis appearing uncomfortable during a meeting with Trump with a caption, "this is pretty much how the whole world sees you".