He describes systematic humiliation, clearly aimed at corroding the humanity of the victims, and which included exposing devout Muslims to insult by prostitutes.

The Centre for the Rehabilitation of Victims of Torture in Copenhagen was the first to provide systematic medical care for torture victims, and to research its effects. "We thought that the aim of torture was to obtain information," states the centre's Dr Inge Genefke. "But no. The main aim of torture is to break down, to destroy the identity, the personality."

After years of research, the centre concluded: "The target group of government-sanctioned torture are leaders of ethnic minorities, human rights fighters, union members, politicians, student leaders, journalists and others."

These were all selected because they were leading personalities, pursuing goals inimical to government policies. Once broken, these victims "are full of anxiety, depressions ... their families suffer. Others are intimidated, afraid of being exposed to the same treatment, and do not dare to follow their more courageous exemplars."

Despite this, many argue that the war on terror justifies such measures. Distinguished columnists have suggested that sleep deprivation or lengthy interrogations, known as "torture-lite", ought to be permissible.

In late 2002, the Washington Post revealed: "Deep in side the forbidden zone at the US-occupied Bagram airbase in Afghanistan ... sits a cluster of shipping containers protected by a triple-layer of concertina wire. The containers hold the most valuable prizes in the war on terrorism - captured al-Qaida operatives and Taliban commanders."

The newspaper went on to describe the process of "rendering", which sends uncooperative prisoners to obedient allies where unsqueamish interrogations are not deemed unlawful. Guantánamo Bay, outside normal legal constraints, holds over 600 detainees who are beyond the reach of judicial processes anywhere in the world.

A rgument continues about whether the US base in the British territory of Diego Garcia is also used for such interrogations. The British government denies this, but the allegations continue, echoed by numerous civil liberties groups.

Now, dictatorship is not the sole instigator of torture. In the age of empire, behaviour that would have aroused universal disapproval in the metropolitan power becomes "necessary" for controlling the the subject nations. Since such behaviour might spread anxiety to the domestic population, who regrettably have votes, it is best confined to unapproachable zones or distant islands under strict military control. In Bagram there is clearly a different law from that which applies in the continental US. Alas, other governments, like the British, find it necessary to condone this.

But this behaviour spreads into the main body politic, and rots civil freedoms there, too. It needs a high-security presence even at home, such as has been established at Belmarsh. It needs fickle and opportunistic politicians, who find difficulty in recognising what were yesterday deemed to be universal, uncontested truths.

The war on terror is a perfect state of psychosis within which the darkness can extend itself. It has no defined boundaries, no fixed territorial enemies: it takes what yesterday were deemed to be simple crimes, and extends them mentally to incriminate whole populations, social groups or religions.

There is only one antidote to this creeping sickness: the insistence upon universal human rights, within whose spirit torture was outlawed by the 1984 UN Convention Against Torture. Forbidden are any officially sanctioned acts "by which severe pain or suffering,whether physical or mental, is intentionally inflicted on a person".

But, forbidden or not, the 3,000 captives accused of sustaining al-Qaida have commonly been fitted with hoods and gags, tied up in positions calculated to cause pain and distress and systematically deprived of sleep.

Jamal al-Harith said: "The whole point of Guantánamo was to get to you psychologically. The beatings were not nearly as bad as the psychological torture - bruises heal after a week, but the other stuff stays with you."

We should join forces with those NGOs which are already demanding that the UN inspect those installations where maltreatment of captives is suspected, and bring such installations under its own jurisdiction.

· Ken Coates is chairman of the Bertrand Russell Peace Foundation and edits its journal, the Spokesman