The round-up of foreign-born terror suspects is described by security sources as "a highly selective operation".

Several weeks ago, MI5 began drawing up a list of Islamic radicals - some linked to al-Qaeda - against whom there was insufficient evidence to bring a criminal case in the UK, but who are regarded as a security threat to Britain and other countries.

Under "normal" circumstances they would be candidates for deportation.

But because they would be subject to torture or even death in their own country, Britain would not take that step - under the terms of the European Human Rights Convention.

The director-general of MI5, Sir Stephen Lander, approved a list of names to be submitted to the Home Office.

Government lawyers then reduced the number of names, taking the view that some of the cases were not strong enough to merit indefinite detention.

A final list was handed to the Home Secretary David Blunkett on 17 December.

He also queried one or two of the names on grounds of evidence and intelligence assessment and they were deleted from the list which formed the basis for Wednesday's dawn raids in London, Bedfordshire and the West Midlands.

A majority of those picked up are from North Africa - a significant number from Algeria, which has long been associated with extremists who have settled in a number of European states.

Uncharted territory

They are being held in high security jails - most of them at Belmarsh in south-east London.

What happens now is exercising lawyers and intelligence experts and takes us into largely uncharted legal territory.

The next stage of the process involves the Special Immigration Appeals Commission (SIAC), a three-person tribunal, chaired by a High Court judge, and including another judge and a lay member. SIAC has the power to grant bail and it is expected that some - or all - of the detainees will apply for bail.

An initial hearing may take place within days.

If anyone is freed as a result such an application it would be a setback for the Home Office and security service but it would prevent them gathering more evidence to justify another detention.

Most of those in custody are not familiar names, bearing out the point that those who have a high profile in the media are of less interest to the security service than those who are working quietly behind the scenes to further the cause of violent Islamic extremism.