Almost 550 detainees from around 40 countries are held at the base.
The last four British men held as terrorism suspects at Guantanamo Bay are expected back in the UK on Tuesday, after almost three years in US custody.
The men, one from Birmingham and three from London, were held after the US accused them of having al-Qaeda links.
Moazzam Begg, Martin Mubanga, Richard Belmar and Feroz Abbasi are expected to be questioned under UK anti-terror laws after their return to RAF Northolt.
The US agreed the men could be released after "complex" talks with the UK.
Britain has provided "security guarantees" to Washington in relation to the men, who were held without trial, but has not said what they include.
Families' 'anguish'
Amnesty International has welcomed the expected "long overdue" return of Mr Begg, from Birmingham, and the other detainees, from London.
We cannot be weak... we need to be there for him
Janette Belmar
Families' hopes and fears
Five other British detainees were freed from Guantanamo last year and were released without charge after questioning by police in the UK.
Some later said they had been hooded, shackled to the floor in painful stress positions and witnessed beatings and other abuse during their time at Guantanamo.
Muslim leaders met Home Office Minister Hazel Blears on Monday to call for the men returning to the UK on Tuesday to be given "urgent medical attention" upon their release.
Moazzam Begg's family has campaigned for his release
Secretary general Iqbal Sacranie told BBC News he understood the men would be examined medically to establish whether they were in a fit state to be questioned.
Insisting they should be released soon, Mr Sacranie said: "What we don't want to do is to apply torture upon torture. They have already been through a lot of suffering."
Lawyers for the four have urged the police to release them immediately.
Louise Christian, who represents Mr Abbasi and Mr Mubanga, believes they should not be arrested.
"It would be a violation of the Human Rights Act to prolong their ordeal," she said.
Richard Belmar was a Catholic but converted to Islam in his teens
Announcing their impending release on 11 January, UK Foreign Secretary Jack Straw said the police and the Crown Prosecution Service (CPS) would decide if they would face prosecution.
BBC Home Affairs Correspondent Danny Shaw said it was expected the men would be arrested by police and questioned when they arrive back in the UK.
It is understood police already had "lengthy dossiers" on each of the men compiled by agents from the UK's security service MI5 who had visited the men nine times at Guantanamo, he said.
The men have gone through the worst possible conditions and it is important that we don't pile more ill-treatment upon them
Muslim Council of Britain
Free - but for how long?
Guantanamo action revealed
He said some of the alleged evidence "may well have been obtained in circumstances not acceptable in courts here, perhaps under duress or perhaps from the battlefields in Afghanistan and so on".
Washington has alleged that all four men trained at camps run by al-Qaeda.
The United States has released about 200 of nearly 800 suspected al-Qaeda and Taleban prisoners held at Guantanamo Bay.
Most of them were captured during the war in Afghanistan in 2001 and 2002.
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In the End, we will remember not the words of our enemies, but the silence of our friends."
- Martin Luther King Jr.(1929-1968)
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