Geoffrey Robertson: This evil despot must be brought to justice

Colonel Gaddafi is the worst man left in the world. His bloodstained record of terrorism, torture and mass murder deserves punishment many times over and a people liberated from the moronic tenets of his Green Book might, if they get the chance, string him up in revenge. But the days of hanging dictators from lampposts are over. International justice must take the place of righteous lynch mobs. Alternatively, if Colonel Gaddafi crushes the revolt, it must ensure that he is hunted down should he set foot again outside Libya. The Security Council, disgracefully slow in responding to this crisis, has an urgent duty either to establish an international court to try Gaddafi or (more simply) to require the prosecutor of the International Criminal Court to investigate and indict him for the massacre of protesters.

The ICC has been cutting its teeth on a few Congo warlords and gnashing them over Sudan's President Omer Hassan al-Bashir (although its indictment might have prompted yesterday's announcement of his early retirement). Its treaty has now been ratified by 114 nations and the irrational hostility of the Bush era has been replaced by cautious support from President Barack Obama. Libya, of course, is not a member of the ICC (and nor are any other Arab countries except Jordan and Djibouti – a situation which new powers in Egypt, Tunisia and elsewhere will be advised to change). But Colonel Gaddafi can be brought within the court's jurisdiction by a Security Council direction – as was Mr al-Bashir when the council referred the situation in Darfur to the ICC prosecutor.

The ICC could not put Colonel Gaddafi on trial for all the international crimes he's committed before 2002, when its jurisdiction commenced, so he would escape retribution for funding and promoting years of terrorism in Europe and Africa, for his assassination campaign against "stray dogs" (several of his opponents were killed in Britain), for the Lockerbie and French airline bombings, and for arbitrary killings of student dissidents. He can, however, be fixed with command responsibility for the crimes against humanity committed by his troops over the past few days as they have shot and killed innocent civilians in a number of towns and cities.

Under Article 7 of the ICC statute, a widespread lethal attack directed against a civilian population amounts to a crime against humanity and the wilful killing of civilians in a civil war amounts additionally to a war crime under Article 8. The Libyan delegation at the UN was wrong to allege it amounts to genocide, since this crime does not apply in the case of armed attacks on political groups.

No doubt they did so in an effort to engage the obligation to intervene, which is imposed on the international community by the Genocide Convention. But this is now a duty that exists under international law whenever it becomes necessary to stop or to punish crimes against humanity. This "responsibility to protect" doctrine has been much touted by the UN, so even if Colonel Gaddafi crushes the revolt and remains head of state, the ICC statute gives him no immunity from prosecution, or from arrest once he has been indicted.

Others should be in the dock beside him – not only the military leaders who ordered troops to fire and planes to bomb, but his own son Saif al-Islam Gaddafi whose mask as Libya's "human face" slipped during his vicious broadcast threat to kill his people "to the last bullet".

This was plainly an incitement to mass murder, which would make him an accomplice to subsequent killings. His indictment would at the very least put an end to travels to his Hampstead mansion or to cocktails on the Mandelson/Rothschild circuit.

I had to cross-examine Saif in a libel action some years ago, when he admitted his fervent support for his father's arbitrary killings and that his so-called "charities" were fronts to arrange compensation for the families of suicide bombers. The UK bears much responsibility for promoting Colonel Gaddafi and son: the money-grubbing LSE gave Saif his doctorate whilst oil money made amoral Labour politicians dance to his tune over the release of the Lockerbie bomber. They were helped, of course, by "useful idiots" – the Scottish politicians who thought they were being compassionate when they were just being stupid.

The Foreign Office even had the Prime Minister sign the most grovelling letter in recent British history, begging Colonel Gaddafi to keep Abdelbaset Ali al-Megrahi's return "low key" – on the 40th anniversary of Colonel Gaddafi's reign! It is astonishing to think that the Crown Prosecution Service recently helped the Gaddafis by putting their opponents here in prison. They were jailed for several years for possessing "information likely to be of assistance to terrorists", namely a blueprint for overthrowing Colonel Gaddafi without harming civilians or visitors.

If indicted, amnesty would not be possible for Gaddafi and son – international law denies it to political and military leaders credibly alleged to bear serious responsibility for crimes against humanity. Any country that gives refuge to fugitive Gaddafis must be made to disgorge them or else pay the severest price in sanctions. If the Colonel regains power but is indicted, his diplomatic status will never again permit him to strut the world and demean it by rants at the UN General Assembly: indictment by an international criminal court is the Achilles' heel of travelling dictators, because it deprives them of the immunity they continue to have against prosecution in the courts of other countries.

So an indictment is the only way to stop states from truckling to an oil-rich, resurgent Gaddafi. It could come either from the ICC or from a UN "special court" set up like the Lebanon tribunal. In either case, immediate action by the Security Council is required. The world should see how the body charged with keeping international peace is doing – or failing to do – its duty.

Geoffrey Robertson QC is a member of the United Nations Justice Council and author of 'Crimes Against Humanity'