The men, who are thought to be mainly young British Muslims of Pakistani origin, were being held in London, with several arrested at a Chinese restaurant in the Borough area of the city.

Detectives were yesterday searching several residential properties across London . . . running into double figures . . . as well as an Islamic school in Mark Cross, near Crowborough, East Sussex.

The men were arrested under the Terrorism Act 2000 after months of surveillance involving Scotland Yard's anti-terrorist branch and MI5. They were in custody at Paddington Green high security police station yesterday.

Security sources said the investigation was focusing on the alleged recruitment and radicalisation of young British Muslims and the facilitation of training for terrorism purposes. It is not yet clear whether the men were part of one group.

Police have only just started the searches and have not ruled out the possibility that the investigation may take them elsewhere.

Sources said there was no evidence that any kind of terror attack was imminent, although police have not disclosed why it was felt necessary to intervene overnight.

"Part of the investigation will focus on the alleged training, recruitment and encouraging of others to take part in terrorist acts, " a source said. "It is very early days and we do not know where this will take us until we do all the searches."

However, another security source suggested those allegedly involved may not just have been recruiting and training suspects, but actually plotting an attack.

There has been no official confirmation of this.

Peter Clarke, head of the anti-terrorist branch, said this weekend that police were trying to keep tabs on "thousands" of people directly or indirectly involved in terrorism in the UK. About 70 counter-terrorism investigations are under way.

A Scotland Yard spokesman said the men had been arrested on suspicion of the commission, preparation or instigation of acts of terrorism. The spokesman said the arrests were not linked to the recent alleged plot to blow up transatlantic airliners or the 7 July bombings in London.

It is thought three of the men were arrested at a restaurant called The Bridge to China Town. The school being searched is an independent school for Muslim boys aged 11 to 16.

In a separate development, two men were arrested in anti-terrorist raids in Manchester.