A Royal Air Force C-17 transport plane carrying the four men touched down at the RAF Northholt base in west London on Tuesday evening.

They were then quickly driven off in a police convoy.

London's Metropolitan Police said in a statement it had arrested Moazzam Begg, Richard Belmar, Martin Mubanga and Feroz Abbasi while they were still sitting on the stationary plane.

They were held under a section of Britain's Terrorism Act 2000 "which refers to the alleged involvement in the commission, preparation or instigation of acts of terrorism", the police said.

Their arrest had been widely expected, but it still comes as a blow to relatives and supporters who have insisted the men, who were detained without trial at the US base in Cuba for as long as three years, are completely innocent.

London detention

The four were taken to London's top security Paddington Green police station. They could face domestic charges but might also be released after questioning on Wednesday.

"They suffered horrendously"

Clive-Stafford-Smith,
lawyer for Begg and Belmar

Five other British men who were freed from Guantanamo Bay last march were formally arrested by police but quizzed only briefly before being allowed to go home.

Begg, Mubanga, Belmar and Abbasi were all arrested in Afghanistan or Pakistan, or had visited there previously and US authorities have linked them to al-Qaida or other terrorist groups.

But the men's lawyers insist they are innocent, saying that they should be set free without delay after their mistreatment in US custody.

"They suffered horrendously," Clive-Stafford-Smith, who represents Begg and Belmar, said.

All four men had spent at least some of their detention in solitary confinement. Begg and Abbasi endured it for nearly two years.