Galloway suffers the slings and arrows as he is hit by 'stress ball'

George Galloway was momentarily dazed by the missile (Reuters)

The latest bizarre episode in the life of George Galloway happened in public yesterday, when a rubber "stress ball" flung from an office window scored a direct hit on the MP as he campaigned in central London.

Mr Galloway, who is standing for a seat in the London Assembly, was on the upper deck of his open-topped battle bus when the missile caught him on the temple, leaving him dazed and bruised.

The ball was believed to have been thrown from the third-floor window of a block on Theobalds Road, Holborn. The police were called and a man was arrested. Scotland Yard said he was being held in custody last night.

Mr Galloway, who has survived expulsion from the Labour Party over his comments on the Iraq war as well as an appearance on Celebrity Big Brother, where he famously pretended to be a cat while wearing a leotard, resumed campaigning after a cup of tea.

Today he is to tour Kingston, Wandsworth and Tooting. Mr Galloway's campaign organisers are said to see yesterday afternoon's incident as a potentially dangerous prank but to have little desire to pursue charges against the alleged perpetrator. The Sun newspaper's website quoted a friend of the arrested man who said it had been a "spur of the moment" act.

Mr Galloway said: "It was, of course, a shock. I have been out campaigning for the past two and a half weeks in the battle bus, and nothing like this has happened.

"We have had an extremely good response from voters and I certainly won't let this deter me from campaigning every day between now and the close of polls for the London Assembly on Thursday next week."

An aide, Kevin Ovenden, who was with Mr Galloway when he was hit, said: "George was on the top of his campaign bus with a dozen people including two very small children, up at the front with a microphone.

"He was hit on the left side of his head and was momentarily dazed. Because of the impact of the blow, he lost his balance and hit the other side of his head on the side of the bus. We stopped the bus and called the police."

Mr Ovenden described the missile as hard, hollow and slightly smaller than a tennis ball, and said it left Mr Galloway with a "nasty bruise" on the side of his head.

Mr Galloway, 53, has been the Respect Party MP for Bethnal Green and Bow since ousting the Labour MP Oona King in the 2005 general election.

Always a controversial figure – he marched alongside Gerry Adams to campaign against British policy in Ireland – Mr Galloway had, throughout the 1990s, argued vociferously for overturning sanctions against Saddam Hussein's regime, and he also visited the dictator in Iraq.

He was expelled from the Labour Party in October 2003 after expressing views that the party chairman, Ian McCartney, said "incited foreign forces to rise up against British troops". He also accused Tony Blair and US President George Bush of acting "like wolves" over Iraq.

Mr Galloway then helped found the anti-war party Respect in 2004. In December that year, he was awarded £150,000 in libel damages from The Daily Telegraph after suing the paper over articles which claimed he had received payments from Saddam Hussein's regime. He denied ever seeking or receiving money from the Iraqi government, and said he had long opposed it.

On his website, he says of his local election campaign: "The Assembly is meant to be London's government but virtually no one has ever heard of anybody on it. I want to change that."