Mr Tatchell and two other members of the gay rights group OutRage! were arrested in central London on Saturday when they pounced on a car in which Mr Mugabe was travelling.

Robert Mugabe: In London on shopping trip

The group - which accuses Mr Mugabe of inciting "anti-gay hatred" - wanted to make a citizen's arrest over the alleged torture of two journalists in Zimbabwe.

The two other protesters involved in the incident outside the Crowne Plaza hotel in Buckingham Gate are expected to be released later.

Mr Tatchell said: "Three of my colleagues blocked the path of his car while I opened the door, grabbed him by the arm, and said `President Mugabe, you are under arrest for torture. Torture is a crime under international law'.

"I turned to the president's security officers and said `Call the police, the president is under arrest on charges of torture'.

The BBC's Jason Kaye: "The protesters say the president should be prosecuted because of 'anti-gay laws' in Zimbabwe"

"Then the police arrived, I asked them to arrest President Mugabe, using the powers in the Criminal Justice Act and United Nations Convention Against Torture, but they ignored our requests."

He said he called on the Attorney General, Lord Williams of Mostyn, to authorise President Mugabe's arrest.

But a spokesman for Lord Williams said: "If Peter Tatchell feels an offence has been committed, he needs to go to the police to ask them to investigate and if necessary arrest the person who he feels has committed the offence. It is not a matter for the attorney general."

'Monstrous'

Mr Mugabe was in the UK on a private shopping trip. He is now returning to Zimbabwe.

Mr Tatchell, who has been involved in more than 1,000 demonstrations over the past 30 years, added: "It is monstrous that Mugabe is going to buy luxuries for his family while millions of people in his country are facing starvation."

The protesters had carried placards bearing slogans including Mugabe Stop Abusing Queers.

President Mugabe has described lesbians and gays as "worse than dogs and pigs" and said they have no constitutional rights.

Earlier this year, Zimbabwe's former president, Canaan Banana, was convicted on charges of sodomy and sexual assault against his male employees.

And the issue of gay rights has returned to the fore in the southern African country as the government draws up a new constitution.

The organisation GALZ - Gays and Lesbians of Zimbabwe - is demanding a constitutional guarantee of protection against discrimination on the grounds of sexual orientation.