Buchwald rejected the argument by Justice Department lawyers that Trump's own First Amendment rights allowed him to block people with whom he did not wish to interact.
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.
"The answer to both questions is no", Buchwald said in her 75-page judgement.
Importantly, the ruling identifies only parts of Trump's account as a public forum subject to First Amendment protections, not the entire account nor the rest of Twitter.
The judge said she disagreed, but that there was no need to get into a "legal thicket" over the issue because she assumed Trump or his social media director Dan Scavino, who also was a defendant in the case, would unblock the users in light of her decision.
Read the full opinion here.
"No government official - including the President - is above the law", wrote Buchwald for the U.S. District Court for the Southern District of NY.
In the lawsuit, the seven individual plaintiffs, which included a University of Maryland professor, a Texas police officer and a NY comic, said they were blocked from the @realDonaldTrump account after posting tweets critical of his policies.
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The Republicans had requested information about an outside informant who spoke to members of the Trump campaign. He also said he still preferred that President Trump not sit for a Mueller interview.
The Justice Department defended Trump's Twitter activity, contending that muting is within the president's "associational freedoms".
The Knight Institute sued Trump last spring on behalf of seven Trump critics who had been blocked on Twitter, arguing that Trump's use of Twitter to announce policy and engage with the public constituted a public forum.
The Knight Institute hopes that the ruling against Trump will set a precedent for other public officials across the country.
On Wednesday the judge agreed with their argument that the social media platform qualifies as a "designated public forum" granted to all US citizens. The Trump administration has not decided yet whether Trump will unblock his critics on Twitter as directed by Buchwald or whether it will appeal the ruling, the New York Times reported.
Noah Feldman, a Harvard law professor, said he thinks the case was wrongly decided and expects it to be reversed.
The judge did, however, state that Trump could mute Twitter users, which remains constitutional as he still has the ability to reply to an account that he has muted. "That is much less restrictive and burdensome on the plaintiffs' speech rights".
Twitter Inc, which is not a party to the lawsuit, declined to comment on the ruling.
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