Archbishop Desmond Tutu entered the conference hall to ululations from the female delegates and a standing ovation.

Diminutive and grinning beneath his snowy white hair, the revered 74-year-old issued a stern warning to educators and politicians alike in the wake of the London bombings: “Freedom requires eternal vigilance.”

He recalled how he felt when, after enduring his entire school life under a segregated system, he arrived at King’s College London to study for a Bachelor of Divinity degree and Masters in theology. “It was like stepping out of a dark room with closed windows and drawn curtains into a glorious sunshine and invigorating fresh air,” he said.

Under apartheid he had been subjected to an intellectual and moral claustrophobia, rote-learning the right answer where an answer was right because a person in authority had declared it to be so. “The goal was to produce those who would not kick against the traces,” he said.

The result was that blatantly unjust policies were created, unchallenged by the elite: from school feeding programmes for white children whose parents could afford school meals while there were none for blacks whose parents could not, to the introduction of detention without trial. Public signs could declare, “Natives and dogs not allowed”, without the privileged class demurring.

At King’s College he discovered a very different world, where students were encouraged to ask awkward questions and look for their own answers, and the press robustly questioned ministers, even lampooning them in programmes such as That Was the Week That Was.

He deplored acts of terrorism such as the London bombings, and urged school leaders to nurture a new breed of human being who will want to be “more compassionate and caring, who will be gentle and sharing”.

But he also launched a stinging attack on George Bush and Tony Blair for their “immoral” war against Iraq and the human rights abuses at Guantanamo Bay.

He told the principals - many of whom had visited Robben Island where Nelson Mandela was held prisoner - that those in power need to be held accountable by a populace that refuses to be cowed into compliance and conformity.

“You have a noble task of helping your charges to become persons who have critical, questioning minds, always ready to ask awkward questions, knowing the difference between authoritative and authoritarian,” he said.

There were light-hearted moments, too. He recalled a visit to a Dutch school to celebrate its 400th anniversary, when a little girl asked him:

“Were you here when the school opened?”

After his speech, delegates sang a Zulu farewell, “Amen Siyakudumisa”, as the Archbishop danced his way down the aisle and out of the hall.