Respect MP George Galloway was taken to hospital with a suspected broken jaw on Friday night after being attacked in the street.
Police arrested a British man over the incident, which is believed to have been prompted by comments Galloway has made about Israel in past weeks.
The 60-year-old was posing for pictures in Notting Hill, London, on Friday evening when a man lashed out at him, his spokesman said. Galloway gave a statement to police after a man aged 39 was arrested.
"George was posing for pictures with people and this guy just attacked him, leapt on him and started punching him," his spokesman said.
"It appears to be connected with his comments about Israel because the guy was shouting about the Holocaust. George is badly bruised but OK. He has bruising to his head and face and is in pretty bad shape."
Police said Galloway was badly injured and was being treated in a London hospital, but was in a stable condition. The arrested man remains in custody.
Last week, the MP was interviewed by police under caution after he was accused of inciting racial hatred by declaring his constituency, Bradford West, an "Israel-free zone".
In a speech he gave in Leeds on 2 August he said: "We don't want any Israeli goods; we don't want any Israeli services; we don't want any Israeli academics coming to the university or the college; we don't even want any Israeli tourists to come to Bradford, even if any of them had thought of doing so."
Galloway called the police investigation into him a "monumental – and monumentally expensive – waste of police time".
A Metropolitan police spokesman said on Friday night: "Police were called at approximately 19.40 hours this evening to Golborne Road, W10, after a man was assaulted in the street. Officers attended. The suspect was found a short time later and stopped. He was arrested on suspicion of ABH and taken to the south London police station where he remains."
In 2008, Galloway was injured after a man threw a rubber stress ball at his head while he was campaigning. A 32-year-old man was cautioned by police over that incident.
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