Why I am saying sorry for London's role in this horror

Next Sunday marks the bicentenary of the abolition of one of history's greatest crimes - the transatlantic slave trade. The British government must formally apologise for it. All attempts to evade this are weasel words. Delay demeans our country. Recalling the slave trade's dimensions will show why. Conservative estimates of the numbers transported are 10-15 million; others range up to 30 million. Deaths started immediately, as many as 5% in prisons before transportation and more than 10% during the voyage - the direct murder of some 2 million people.

Conditions imposed on survivors were unimaginable. Virginia made it lawful "to kill and destroy such negroes" who "absent themselves from ... service". Branding and rape were commonplace. A Jamaican planter, Thomas Thistlewood, in 1756 had a slave "well flogged and pickled, then made Hector shit in his mouth" for eating sugar cane. From 1707, punishment for rebellion included "nailing them to the ground" and "applying fire by degrees from the feet and hands, burning them gradually up to the head".

When in 1736 Antigua found there was to be a rebellion, five ringleaders were broken on the wheel, 77 burned to death, six hung in cages to die of thirst. For "lesser" crimes, castration or chopping off half the foot were used. A manual noted: "Terror must operate to keep them in subjection."

Barbarism's consequences were clear. More than 1.5 million slaves were taken to the British Caribbean islands in the 18th century, but by its end there were only 600,000. By 1820, more than 10 million Africans had been transported across the Atlantic and 2 million Europeans had moved. But the European population grew to 12 million while the black slave population shrank to 6 million.

If the murder of millions, and torture of millions more, is not "a crime against humanity", these words have no meaning. To justify murder and torture on an industrial scale, black people had to be declared inferior, or not human. As historian James Walvin noted, there was a "form of bondage which, from an early date, was highly racialised. By 1750, to be black in the Americas (and often in Europe) was to be enslaved." The 1774 History of Jamaica argued black slaves were a different species, able to work "in a very bungling and slovenly manner, perhaps not better than an orangutan".

Material being produced today to mark the anniversary of the abolition of the slave trade makes it appear that white people liberated black - the assumption being they could not do it themselves. In reality, slaves rose against the trade from its inception. This broke it.

The first recorded slave revolt was in 1570. There were at least 250 shipboard rebellions. Jamaican slave society faced a serious revolt every decade, in addition to prolonged guerrilla war. In 1760, 30,000 Jamaican slaves revolted. The culmination, recorded in CLR James's magisterial The Black Jacobins, was the 1791 slave revolt in St Domingue. After abolition of the trade, slavery in British possessions was abolished following revolts in Barbados in 1816, Demerara in 1823, and Jamaica in 1831, in which 60,000 slaves participated. For this reason Unesco officially marks August 23, the anniversary of the St Domingue rebellion's outbreak, as slavery's official remembrance day.

No one denigrates William Wilberforce, but it was black resistance and economic development that destroyed slavery, not white philanthropy.

Slavery's reality is increasingly acknowledged outside Britain. One of the few things on which I agree with George Bush is his description of transatlantic slavery as "one of the greatest crimes of history". The Virginia general assembly last month expressed "profound regret" for its role, stating slavery "ranks as the most horrendous of all depredations of human rights and violations of our founding ideals". The French national assembly declared slavery a "crime against humanity". In 1999, Liverpool became the first major British slaving city to formally apologise. The Church of England Synod followed suit.

The British government's refusal of such an apology is squalid. Until recently, almost unbelievably, it refused even to recognise the slave trade as a crime against humanity, on the grounds that it was legal at the time. It helped block an EU apology for slavery.

Two arguments are brought forward against official apology - not only by the government but by David Cameron. First, an apology is unnecessary because this happened a long time ago. This would only apply if there had been a previously apology - there hasn't been. Slavery was the mass murder of millions of people. Germany apologised for the Holocaust. We must for the slave trade.

Second, that apologising is "national self-hate". This is nonsense. Love of one's country and its achievements is based on reality, not denying it. A Britain that contributed Shakespeare, Newton and Darwin to human civilisation need fear comparison with no one. A British state that refuses to apologise for a crime on such a gigantic scale as the slave trade merely lowers our country in the opinion of the world.

It is for that reason that I invite all representatives of London society to join me in following the example of Virginia, France, Liverpool and the Church of England, by formally apologising for London's role in this monstrous crime.

Ken Livingstone, the mayor of London.