The last Britons held as terror suspects at the US jail at Guantanamo Bay for up to three years arrived home on Tuesday and were immediately arrested under Britain's domestic anti-terrorism laws.
A Royal Air Force C-17 transport plane carrying the four men touched down at the RAF Northolt base in west London just after 5pm, where they were whisked 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 of 2000 "which refers to the alleged involvement in the commission, preparation or instigation of acts of terrorism," 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.
Britain's top anti-terrorist policeman, Detective Assistant Commissioner Peter Clarke, said on Tuesday his force had discussed the case with Muslim groups and recognized there were "strong feelings" about the case.
"But the fact is that we have an absolute duty on behalf of all communities to investigate the circumstances leading to the men's detention," he said, insisting they would be treated "properly and fairly."
The men remained jailed late on Tuesday at London's top-security Paddington Green police station, the usual holding point for terrorism suspects, and were likely to be held overnight. They were to be questioned yesterday.
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, 36, Mubanga, 31, and Belmar and Abbasi, both 25, were all arrested in Afghanistan or Pakistan, or had visited there previously, and US authorities have linked them to al-Qaeda or other terrorist groups.
However, the men's lawyers insist they are innocent, saying on Tuesday that they should be set free without delay after their alleged mistreatment in US custody.
"They suffered horrendously," said Clive Stafford-Smith, who represents Begg and Belmar.
"Given what they've been through, they've held up as well as can be expected but they're victims of torture and torture victims suffer from PTSD [post-traumatic stress disorder]," he said.
"The British would be totally insane to put them on trial," he told the BBC later on Tuesday, adding that his clients would definitely be bringing a case against the US government.
All four men had spent at least some of their detention in solitary confinement, said Stafford-Smith and lawyer Louise Christian, who represents the other pair, with Begg and Abbassi enduring it for nearly two years.
Christian said she was "apprehensive" about her clients' health.
"Martin Mubanga has written letters alleging extremes of hot and cold, being deprived of basic sanitary items, such as toilet paper and toothpaste," she said.