Police are investigating after Labour activist Owen Jones was “kicked in the head” by a group of men outside a pub.
The Guardian columnist said he had been celebrating his birthday with friends when the men made a “direct beeline” for him in the early hours of Saturday.
Labour leader Jeremy Corbyn said an attack on a journalist was “an attack on free speech and our fundamental values”.
The Met said it was working to establish the “full circumstances”.
Mr Jones told the BBC he had spent Friday night with his partner and some friends at the Lexington pub on Pentonville Road, Islington.
The six of them left the pub at about 02:00 BST on Saturday. As they were saying goodbye to each other, a group of “three or four” men made a “direct beeline” for Mr Jones.
Mr Jones claimed it was a “premeditated” move by “far right” activists.
“[They] marched over, kicked me in the back, threw me on the floor… and then kicked me in the head,” he said.
Mr Jones said his friends were also punched in the head as they tried to stop the “frenzied” attack on him – and that there was “no question” the men were “specifically” targeting him.
In response to the incident, Mr Corbyn tweeted: “Owen believes it was politically motivated, and we know the far right is on the march in our country.
“An attack on a journalist is an attack on free speech and our fundamental values.”
A spokesman for the Metropolitan Police confirmed it had spoken to witnesses and would review CCTV footage.
No arrests have been made and the investigation continues.
The Lexington’s general manager, Chris Smith, said he had looked back at the venue’s CCTV and thought it was “possible” the suspects were caught on footage.
He said his staff were “all upset” that Mr Jones had been targeted and would “do everything” to aid the police investigation.
Mr Jones said the attack was the latest in a series of assaults since he was verbally abused by protesters on College Green in Westminster earlier this year.
“The far right is an increasingly scary, violent and threatening menace in this country and other countries as well,” he said.
Mr Jones said he would not let far right groups “scare or intimidate” him.
But he added: “I’m obviously very concerned and worried – not simply about my own personal safety, but of others, and the fact that the far right feels increasingly emboldened and far right-types are feeling increasingly prepared to resort to thuggery and violence.”
Shadow attorney general Shami Chakrabarti said: “This is a very febrile moment in Britain – and I’m sure that everyone, regardless of their politics, will want to send their solidarity to Owen Jones.”
“The police will no doubt be concerned to investigate any political and homophobic motives behind what may be a very serious hate crime,” she added.