Enter the incongruous figure of Judge Hamoud Abdulhamid al-Hitar, dressed in a long black robe, a ceremonial dagger at his waist and a copy of the Guardian under his arm.

A judge in the high court of Yemen, he had been invited to London by the British government because the Foreign Office, the attorney general and the Metropolitan police, not to mention several Muslim organisations, all wanted to know about his unusual method of fighting terrorism - by theological dialogue.

In the aftermath of the September 11 attacks on New York and Washington, the Yemeni authorities, under considerable pressure from the United States, rounded up several hundred suspected troublemakers and kept them in jail without trial. Many, the authorities readily admit, had not committed any crime but were known as sympathisers - if not active supporters - of Osama bin Laden.

The approach pioneered by Judge Hitar, who is also chairman of the Yemeni Human Rights Organisation, is to "re-educate" and release them, subject to guarantees of good behaviour. The success rate in re-education is about 90%, according to Judge Hitar, and more than 100 have been freed so far.

The basic idea is very simple: that Islamic militants are not fundamentally bad people but have mistaken views of Islam that can be corrected through religious argument based on the Koran and the teachings of the prophet (the Sunna). If they can be genuinely convinced of their error, they will not commit criminal acts and - perhaps more importantly - will not encourage others to do so either.

"It's similar to the work of a doctor," Judge Hitar said. "We diagnose and we treat it."

The theological dialogue committee, which Judge Hitar chairs, was set up in September 2002 to hold discussions with returnees from Afghanistan and "youths holding extreme theological ideologies contradictory to the unanimity of Islamic scholars". (Since the Yemeni justice system is based on Islamic law, judges are well qualified to deal with the religious issues.)

"We chose a group of these young people, and picked the most extreme and the best educated," Judge Hitar said. "They asked why we had come. We said it was to carry out a dialogue and our number one point of reference would be the Koran and the Sunna. "We told them: 'If YOU are right we will follow you, but if WE are right you should follow us'.

"We set out an agenda by finding out what problems they suffered from and what [religious] authorities they depended on." Two particular attitudes among the militants soon came to the fore: a belief that they could kill non-Muslims in the name of jihad, and a belief that Yemen's political system was contrary to Islam.

"They considered Yemen as a non-Islamic state," he said. "They said the government is not governing according to God's book and is pro-western. Some of them were not aware of the treaties and agreements between Yemen and other countries. Some thought it was acceptable to shed the blood of non-Muslims anywhere on earth."

The secret of the dialogue's apparent success is that it confronts such views in Islamic terms, citing the Koran and other authorities that the militants respect. It makes use, for example of the constitution of Madina, established in the prophet's time, which protected the rights of Jews and other non-Muslims.

Yemen's international relations, Judge Hitar explained, are based on agreements and governed by the United Nations - arrangements which are fully compatible with the Koran and the Sunna.

"UN signatories are in a state of peace with one another, and it is not permissible to attack citizens from other UN members. [Non-Muslim] people visiting Yemen have a visa, so it is not permissible to attack them or force them to convert to Islam."

Those who accept the re-education are asked to sign a statement condemning violence, committing themselves to obey the law and dissociating themselves from al-Qaida or any armed group. They must also agree to respect the rights of non-Muslims and promise not to attack the interests of friendly states.

But are these changes of heart genuine, or are the militants simply grabbing a "get-out-of-jail-free" card? That is a question Judge Hitar is often asked.

"These young people are not going to be convinced easily," he answered. "They don't agree just to get out of jail. One of them said 'I am prepared to stay for 20 years in jail to prove that I have changed.'"

Those released are kept under surveillance, and Judge Hitar emphasises that suspects who have been charged with actual crimes have to be brought before the courts.

"Dialogue is for everyone but release is for the innocent," he said. The principle of dialogue, he believes, is one that can be used to deal with Islamic militancy far beyond Yemen, even in Britain which he describes as "the capital of religious tolerance".

"With total confidence I think this method can be applied in the UK," he said. "There are British Muslims who have the ability [to conduct dialogues] and we are prepared to train them further. We are ready to co-operate with them and train them in Yemen or in Britain."

It was time for Judge Hitar to leave. The BBC's Arabic Service were eager to interview him, and later he would be meeting Muslims in Birmingham. Preparing to step out, once again, into the British winter, he held up a ballpoint pen and waved goodbye with it.