NEW YORK — President Trump’s decision to block his Twitter followers for their political views is a violation of the First Amendment, a federal judge ruled Wednesday, saying that Trump’s effort to silence his critics is not permissible under the U.S. Constitution because the digital space in which he engages with constituents is a public forum.

The ruling rejects administration arguments that the First Amendment does not apply to Trump in this case because he was acting as a private individual. In a 75-page decision, Judge Naomi Buchwald said Trump, as a federal official, is not exempt from constitutional obligations to refrain from “viewpoint discrimination.”

“No government official — including the President — is above the law,” wrote Buchwald for the U.S. District Court for the Southern District of New York.

The decision marks a victory for free-speech activists representing seven Twitter users who alleged that their rights had been infringed after they tweeted at Trump critiquing his policies. Trump blocked them on Twitter, preventing them from seeing his tweets or interacting with them.

“We are extremely pleased that the judge held that the president’s blocking of critics from the @realDonaldTrump Twitter account violates the First Amendment,” Katie Fallow, a senior staff attorney with the Knight First Amendment Institute who argued the case, said.

As public officials increasingly use social media to engage with constituents and shape public debate, Fallow said it’s important that they heed this ruling and take care not to suppress speech on the basis of viewpoint. That has been a longstanding rule established well before the advent of popular social media networks like Facebook and Twitter, she said. “If you could exclude all your critics, you can create an echo chamber, a one-sided picture of the debate.”

The government does not dispute that Trump blocked the Twitter users for political reasons. But the Justice Department had argued Trump was largely acting in a personal capacity, much like “giving a toast at a wedding or giving a speech at a fundraiser.”

But through his Twitter bio and the way in which he frequently uses the medium to comment on public policy, Trump portrays his account as presidential “and, more importantly, uses the account to take actions that can be taken only by the President as President,” Buchwald wrote.

What’s more, Buchwald said, the space below Trump’s tweets that show the public’s replies are a public forum, because it is “generally accessible to the public” and anyone with a Twitter account is able to view those responses, assuming that the user has not been blocked.

While presidents retain their own First Amendment rights even when they take public office, the judge said, Trump “cannot exercise those rights in a way that infringes the corresponding First Amendment rights of those who have criticized him.”

Twitter has long been dogged by questions about how far its users’ right to speech may extend. In the past, its own executives have described the company as being “the free speech wing of the free speech party,” holding that Twitter takes no position on the messages posted by its users.

But the rise of online bullying, hate speech and harassment on Twitter’s platform has forced the company to confront its insistence on neutrality. Last year, the company unveiled new policies to address threats of violence or reports of abuse.

The White House did not immediately respond to a request for comment. Twitter declined to comment.