British sailors and marines freed by Iran feared they would be executed during their captivity, they said yesterday.
The 15 sent condolences to the families of four UK troops and a civilian interpreter killed in Iraq on Thursday.
And they gave details of the "constant psychological pressure" to which they said they were subjected by Iranian authorities.
They said they were blindfolded, bound, heard weapons being cocked, and were told if they did not admit they had strayed into Iranian waters they faced seven years in prison.
They said they were 1.7 nautical miles from Iranian waters when they were taken.
Two of the 15 read a statement at Royal Marines Base at Chivenor, north Devon, then others answered questions.
Perhaps the most chilling description came from Royal Marine Joe Tindell, 21, from Shooters Hill, south east London, who told BBC News 24: "On day two the mood completely changed, they changed from the military dress to all black, their faces covered.
"We thought we were going to the British embassy but we got taken to a detention centre, all 15 of us.
"We had a blindfold and plastic cuffs, hands behind our backs, heads against the wall. Basically there were weapons cocking. Someone, I'm not sure who, someone said, I quote `lads, lads I think we're going to get executed'.
"From there we were rushed to a room, quick photo and then stuffed into a cell and didn't see or speak to anyone for six days."
The seven marines and eight sailors were seized on March 23 as they carried out a routine search of a cargo ship in the northern Gulf.
The 15, who include one woman, Leading Seaman Faye Turney, began their statement by sending condolences to the families of the five people killed in Iraq.
Lieutenant Felix Carman, 26, of Swansea, south Wales, said: "I can clearly state we were 1.7 nautical miles from Iranian waters." Royal Marine Captain Chris Air, 25, from Altrincham, Cheshire, said they saw two speedboats approaching rapidly about 400m away.
"One officer spoke good English and I explained that we were conducting a routine operation, as allowed under a UN mandate.
"But when we tried to leave, they prevented us by blocking us in.
"Another six boats were closing in on us ... had we resisted there would have been a major fight, one we could not have won, with consequences that would have had major strategic impact.
"We were interrogated most nights, and presented with two options.
"If we admitted we had strayed, we would be on a plane back to the UK soon. If we didn't we faced up to seven years in prison."
Capt Air said: "Let me be absolutely clear, from the outset it was very apparent that fighting back was simply not an option. Had we chosen to do so then many of us would not be standing here today. Of that I have no doubts."