Some of the most dangerous fighters captured thus far in the war on terror awoke to their first full day of lock-up in Guantanamo Bay Saturday, which the head of security said was locked tight thanks to "layered" security.

Extreme measures are being taken at the base, with additional guards in place to watch the 20 Al Qaeda and Taliban prisoners.

With their hands tied and an American soldier on each side, the detainees were permitted an exercise walk Saturday, U.S. officials said.

"This was sort of our test run. It went extremely well," Army Col. Terry Carrico said. "There's going to be no break out. I'm very confident."

Before falling to sleep Friday night on mats, some of the terror prisoners prayed.

Although it rained overnight the prisoners stayed dry thanks to the metal roofing on their chain-link cells. The individual outdoor units stayed lit with halogen floodlights, which stayed on to keep those inside visible to their guards.

The 20 are the first group of Al Qaeda and Taliban prisoners to be brought from Afghanistan to Guantanamo Bay, on the eastern tip of Cuba, where they will face intense interrogation, especially concerning the whereabouts of terror suspect Usama bin Laden.

In London, the British Foreign Office said Saturday that it had been informed by the Americans that a British citizen was among the 20 prisoners. A Foreign Office spokeswoman said officials were trying to confirm his identity.

The U.S. military has clamped down with heavy security, saying the fighters have shown they're willing to kill themselves and their captors. In November, a prison mutiny in northern Afghanistan took three days to put down, left as many as 450 fighters dead and cost the life of a CIA agent.

Reporters at the U.S. base weren't being allowed to visit the newly built detention center. Carrico would not comment on when the next group from Afghanistan was expected to head here.

On Saturday, the prisoners were to be given meals and showers, and be allowed to walk around outside their cells. But during the exercise walk, each was to have his hands bands and a military police guard at each side, Carrico said.

Carrico said some of the prisoners arrived wearing turquoise surgical masks after initial tests suggested the presence of tuberculosis. Those prisoners were to undergo chest X-rays to confirm whether they were infected, he said.

Each prisoner will be given a Quran as a "comfort item" if he doesn't have one, Carrico said. Each also receives two bath towels — one for a prayer mat and the other for showering — a washcloth, toothpaste, toothbrush, soap and shampoo.

An Air Force C-141 cargo plane touched down with the prisoners Friday afternoon. The shackled men were led off the plane and into buses by more than 50 Marines. Several detainees appeared to struggle with the Marines, and two were forced to their knees on the tarmac before being allowed to stand again and walk to the buses. At least one prisoner had been sedated on the flight from Afghanistan.

The buses took the men to a ferry, which carried them across Guantanamo Bay to the detention camp.

The camp has room for 100 prisoners now and soon could house 220. A more permanent site under construction is expected to house up to 2,000.

The Red Cross and other groups are to monitor conditions, amid worries by some human rights groups that the heavy security measures violate the prisoners' rights and that U.S. officials plan military tribunals for the prisoners with lowered standards of due process.

U.S. officials insist conditions do not violate human rights.

The United States is reserving the right to try Al Qaeda and Taliban captives on its own terms and is not calling them "prisoners of war," a designation that would invoke the Geneva Convention.

The Guantanamo base is one of America's oldest overseas outposts. The U.S. military first seized Guantanamo Bay in 1898 during the Spanish-American War.

The Associated Press contributed to this report.