US President Donald Trump’s practice of blocking Twitter users who publicly criticise him has been banned by a federal judge who ruled the practice breaches US free speech laws.

Judge Naomi Reice Buchwald in a Manhattan court ruled on Thursday morning (AEST) that Mr Trump may not legally block Twitter users because doing so violates their rights under the First Amendment of the US Constitution.

The ruling came in a case brought by seven Twitter users who had been blocked by the @realDonaldTrump account, which has more than 52.5 million followers, after they criticised the President.

The users were joined by the Knight First Amendment Institute at Columbia University, which contended that the discussions arising from Mr Trump’s tweets should be considered a public forum.

Ms Buchwald rejected an argument made by Justice Department lawyers that Mr Trump’s own First Amendment rights allowed him to block people with whom he did not wish to interact.

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 Mr Trump has blocked 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 among others.

In addition to Mr Trump, the lawsuit named the President’s social media director, Dan Scavino, as a defendant.

The Knight Institute and the individual Twitter users claimed in their lawsuit that by blocking users for their views, Mr Trump was shutting them out of discussion in a public forum, violating the First Amendment.

“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,” Ms Buchwald said.

She stopped short of directly ordering Mr Trump to unblock users, saying it was not necessary to enter a “legal thicket” involving courts’ power over the President.

“Because no government official is above the law and because all government officials are presumed to follow the law once the judiciary has said what the law is, we must assume that the President and Scavino will remedy the blocking we have held to be unconstitutional,” she wrote.

Mr Trump was a prolific tweeter from his @RealDonaldTrump account even before being elected in 2016 and has since made it an integral and controversial part of his presidency.

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

A spokesman for the US Department of Justice, which represents the president in the case, had no immediate comment. Twitter also has yet to comment on the ruling.

-with AAP