Peter Tatchell said he had received "bashings" which incapacitated him
Human rights campaigner Peter Tatchell has quit as a parliamentary candidate for the Green Party, blaming brain injuries received during protests.
Mr Tatchell, who was to contest Oxford East at the next election, said his health was "not strong enough" to do the job of MP properly.
He blamed "bashings" received at a rally in Moscow and from Zimbabwe President Robert Mugabe's bodyguards.
Mr Tatchell said he had received further injuries in a crash in July.
In a statement, the 57-year-old said: "It is with great regret and reluctance that I am standing down as the Green Party parliamentary candidate for Oxford East.
"My brain injuries from the Mugabe and Moscow bashings mean that I would not be able to campaign effectively in the general election or do the duties required of an MP, if I was elected.
"It would not be right for me to seek election if I could not do the job of an MP to the high standards that I want and that Oxford East voters have a right to expect.
"If I was elected, I could manage the parliamentary duties or the constituency work. But my health is not strong enough for me to do both."
He said his injuries had been exacerbated by an accident in July when he was travelling on a bus which swerved sharply and hit his head on the handrail.
"My memory, concentration, balance and coordination have been adversely affected. I can't campaign at the pace I used to," he said.
Mr Tatchell was selected as the Green candidate for Oxford East in April 2007 and arrested the next month at a gay pride march in Moscow.
He was also injured when trying to make a citizen's arrest of Mr Mugabe in Brussels six years earlier.
After pulling out of the election, Mr Tatchell said: "This is huge disappointment and frustration. Oxford East is a target Green seat."
Mr Tatchell, a prominent gay rights campaigner, has supported the Green Party since 1988.
He contested the Bermondsey by-election in 1983 for Labour, losing to Liberal Simon Hughes in a bitter battle in which there were accusations of homophobia, after the Liberals used the term a "straight choice".