I had President Mugabe of Zimbabwe under arrest in October 1999. As his limousine left his central London hotel, my three OutRage! colleagues ran into the road, forcing his motorcade to halt. I ran from behind, opened Mugabe's car door, grabbed him, read the charge of torture, and summoned the police. Despite presenting evidence for his arrest under Britain's anti-torture laws, we were arrested and Mugabe was set free. The government had a chance to put the Zimbabwean leader on trial. But instead of prosecuting him, it gave Mugabe full legal and diplomatic protection.

Now, three years later, Tony Blair and Jack Straw condemn President Mugabe's tyranny and demand action against his regime. But in 1999, when they had an opportunity to arrest him – which might have ended his abusive rule and saved many lives – they refused.

It took until March this year for Zimbabwe to be suspended from the Commonwealth. Although symbolically important, suspension is a soft, easy option. It does nothing to undermine Mugabe. His abuses are continuing, and his people are still suffering.

If Britain and the rest of the world were really serious about challenging Mugabe they would issue warrants for his arrest on charges of torture, under the UN Convention Against Torture 1984. That would really scare him.

The issuing of warrants could lead, eventually, to Mugabe being put on trial, like Slobodan Milosevic. It might also act as a restraint on his repression. If he thought that one day he might be prosecuted, it could make him think twice about authorising further atrocities.

Charges under the UN Convention Against Torture could be based on any of the hundreds of instances of state-sanctioned torture documented by Zimbabwe's human rights watchdog, the Amani Trust. The UN Convention has been ratified by over 120 countries, including Britain. These countries have incorporated the Convention into their own domestic legislation, such as Section 134 of the UK's Criminal Justice Act 1988. It requires signatory states to arrest any person present on their territory, where there is evidence they have committed or authorised torture, regardless of where in the world this torture occurred.

Although most countries have ratified the UN Convention Against Torture, none are enforcing it. President Mugabe is free to travel around the world as he pleases. In the past few months, he has made trips to several African and Asian states.

Last September, when I planned to go to Australia to seek Mugabe's arrest at the Commonwealth summit, I was banned from entering Australia for five months. The Immigration Minister, Philip Ruddock, would not let me return to the country where I was born and where all my family live. Classified as a threat to public order because of my bid to have Mugabe arrested, I was not allowed to visit my dying stepfather. When he died, I was prevented from attending his funeral in Melbourne. Yet Mugabe was free to visit Australia, despite his grisly human rights record.

Journalists tortured

There is ample evidence for President Mugabe's arrest. Ray Choto and Mark Chavunduka, two black journalists on The Standard newspaper in Harare, are well-documented torture victims. I have sworn affidavits from these men, attesting to their torture in 1999. Their affidavits are backed by Amnesty International and corroborated by the Zimbabwe High Court.

According to Amnesty International: "Military interrogators beat both men all over their bodies with fists, wooden planks and rubber sticks, particularly on the soles of their feet, and gave them electric shocks all over the body, including the genitals".

Choto and Chavunduka say their interrogators told them they were being tortured on Mugabe's personal orders. President Mugabe has since refused publicly to condemn their torture – and has appeared to endorse it, implying that the two men got what they deserved.

These affidavits provide the legal basis for Britain – and other countries – to issue warrants for the Zimbabwean leader's arrest.

To excuse their inaction, some governments claim that President Mugabe has immunity from prosecution because he is a Head of State. But according to the UN Convention Against Torture, there are no exemptions. No one is immune, not even serving Heads of State.

Over 50 years ago, following the Nazi atrocities, the Nuremberg Tribunal verdicts established the international human rights principle that in cases of crimes against humanity, such as torture, nobody is above the law.

This Nuremberg principle still applies. Under Article 27 of the UN Rome Statute 1998, signed by 139 countries (including Britain), Heads of State are not exempt from criminal responsibility for acts of torture. This principle has been since reiterated in the case of Slobodan Milosevic, who is currently on trial in The Hague.

He was initially indicted for crimes against humanity in 1999, while he was Head of State of Yugoslavia. It was recognised by the international tribunal in The Hague that a Head of State does not have immunity from prosecution for grave human rights abuses.

The indictment, arrest and prosecution of Milosevic sets a contemporary precedent for the arrest and trial of the Zimbabwean President. If Slobodan Milosevic can stand trial in The Hague, why can't Robert Mugabe?

More information about Peter Tatchell's human rights campaigns: www.petertatchell.net. Donations: send cheques to "Peter Tatchell Human Rights Fund", PO Box 35253, London E1 4YF