Donald Trump is being sued by Twitter users who claim he and his team have blocked them in an "unconstitutional" move.

The President is a big fan of the social media site and often uses it to express his forthright views, leading some to disagree with him and attack him online.

Now, seven tweeters allege the US leader and his aides have violated the country's First Amendment - which guarantees the rights of free expression and speech - by blocking them just because they have criticised him or his policies.

People who use the site are unable to see or respond to tweets from accounts that have blocked them.

The lawsuit was filed by a free speech group at Columbia University called the Knight First Amendment Institute, which alleges the @realDonaldTrump account is trying to suppress dissent.

The institute wants the Supreme Court to rule that blocking people from the account is unconstitutional.

The court said last month that social media platforms like Twitter have provided "perhaps the most powerful mechanisms available to a private citizen to make his or her voice heard", allowing people to "petition their elected representatives and otherwise engage with them in a direct manner".

In a statement, the institute argues Mr Trump's account is a "public forum" under the First Amendment, meaning the US government cannot exclude people from it simply because of their views.

It also contends the White House is violating the seven users' First Amendment right to petition their government for redress of grievances.

Jameel Jaffer, the Knight Institute's executive director, said: "The First Amendment applies to this digital forum in the same way it applies to town halls and open school board meetings.

"The White House acts unlawfully when it excludes people from this forum simply because they've disagreed with the President."

University professor Philip Cohen, one of the seven people suing Mr Trump, said: "Being blocked by Trump diminished my ability to respond and engage in the political process. There has been measurable impact on my ability to be heard.

"Yes, I can still say what I want to say, but not to those I want to speak to, when I want to say it, or in the way that means the most to me. It's disempowering to be prohibited from speaking."

The President's Twitter account has 33.7m followers. The official @POTUS account has 19.3m.

The institute sent a letter to the White House last month suggesting it would file a lawsuit if the President and his aides did not unblock individuals. It claims that the administration did not respond.