Aides reportedly have tried to rein in his tweeting, which often starts early in the morning. But he has remained unfettered and used Twitter to promote his agenda, announce policy and attack critics, especially the media, and the investigation into possible Russian connections with his campaign.

The plaintiffs claimed in their lawsuit that by blocking users for their views, Trump was shutting them out of discussion in a public forum, violating the First Amendment.

When one Twitter user blocks another, the blocked user may not respond to the blocker's tweets on the social media platform.

Media reports say among those Trump has blocked are novelists Stephen King and Anne Rice, comedian Rosie O'Donnell, model Chrissy Teigen, actress Marina Sirtis and the military veterans political action committee VoteVets.org.

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The individual plaintiffs in the lawsuit include Philip Cohen, a sociology professor at the University of Maryland; Holly Figueroa, described in the complaint as a political organiser and songwriter in Washington State; and Brandon Neely, a Texas police officer.

Buchwald agreed with the plaintiffs' contention that the discussions arising from Trump's tweets should be considered a public forum. She rejected an argument made by Justice Department lawyers that Trump's own First Amendment rights allowed him to block people with whom he did not wish to interact.

"While we must recognise, and are sensitive to, the President’s personal First Amendment rights, he cannot exercise those rights in a way that infringes the corresponding First Amendment rights of those who have criticised him," Buchwald said.

Buchwald said Trump could "mute" users, meaning he would not see their tweets while they could still respond to his, without violating their free speech rights.