DONALD Trump has been fiercely criticised for praising a Republican congressman who body-slammed a journalist, with critics claiming it risks encouraging further attacks.

The US president called Greg Gianforte a “tough cookie” and said he thinks the incident helped the party win the election in Montana.

The congressman pleaded guilty to misdemeanour assault after attacking Guardian reporter Ben Jacobs the evening before he won a 2017 special election to serve the remaining 18 months in the House term, vacated by now-interior secretary Ryan Zinke.

“Any guy that can do a body-slam – he’s my kind of guy,” Trump told a laughing and cheering crowd at a campaign rally in Missoula, Montana, on Thursday.

“He’s a great guy, tough cookie.”

The Guardian’s US editor, John Mulholland, said in a statement: “The president of the United States tonight applauded the assault on an American journalist who works for the Guardian. To celebrate an attack on a journalist who was simply doing his job is an attack on the First Amendment by someone who has taken an oath to defend it.

“In the aftermath of the murder of Jamal Khashoggi, it runs the risk of inviting other assaults on journalists both here and across the world where they often face far greater threats. We hope decent people will denounce these comments and that the president will see fit to apologise for them.”

Witnesses to the incident in May last year said Gianforte became enraged over coverage he perceived as biased before throwing Jacobs to the ground and punching him.

Gianforte initially falsely claimed the reporter had grabbed him by the wrist and pulled both of them to the ground, according to documents released under a court order following media requests.

In his speech, Trump reflected on hearing about the incident while in Rome meeting with world leaders. “I said ‘Oh, this is terrible. He’s going to lose the election’. And then I said ‘Well, wait a minute. I know Montana pretty well. I think it might help him’. And it did.”

A Downing Street spokeswoman declined to comment on Trump’s speech, but said that “any violence or intimidation against a journalist is completely unacceptable”.

Courtney Radsch, advocacy director for the CPJ, commented: “We are disturbed once again to see president Trump standing up for those who would attack the press and contributing to an environment in which journalists are feeling less safe and in which attacks against journalists appear to have become acceptable.”

Gianforte admitted a misdemeanour assault charge in June 2017 and said in an apology letter that he alone was responsible for the attack.

He paid a $385 fine, completed 40 hours of community service and 20 hours of anger management counselling, and donated $50,000 to the CPJ.