But Iran has denied claims of threats, isolation and 'psychological pressure' during the 13-day stand-off.
The 15 British sailors and marines seized by Iran in the Gulf last month said on Friday they were blindfolded, bound, kept in isolation and warned that they faced up to seven years in jail.
"Throughout our ordeal, we faced constant psychological pressure," they said in a joint statement at a news conference.
Iran said they were detained for entering its waters illegally but Britain said they were in Iraqi waters.
"We were interrogated most nights and given two options. If we admitted that we had strayed, we would be back on a plane to the UK pretty soon. If we didn't, we faced up to seven years in prison," the statement said.
Iran said Friday's news conference was "theatrical propaganda" to cover up their illegal entry into its waters.
Before being freed, Iranian TV showed some of the 15 saying they had entered Iranian territory and had been well treated.
But on Friday the 15 said they heard weapons being cocked behind them after their capture and feared the worst.
The sole woman, Faye Turney, was kept isolated for several days and told by her captors that the others had been sent home.
Iran freed the sailors and marines on Thursday after a 13-day stand-off and they flew home to an emotional reunion with their families.
Their description of captivity was in sharp contrast to the images of them smiling on Iranian television which they now say were an Iranian "media stunt".
Tehran said the same about Friday's news conference.
"Such staged moves cannot cover up the mistake made by British military personnel who illegally entered Iran's territory," said a Foreign Ministry statement. "Such theatrical propaganda can not justify the soldiers' mistake."
The sailors said that after their arrest in the Gulf, they were taken to a prison in Tehran.
"We were blindfolded, our hands were bound, we were forced up against a wall," they said.
They were "stripped and dressed in pyjamas ... we were kept in stone cells, approximately eight feet by six, sleeping on piles of blankets. All of us were kept in isolation."
They said they had been arrested in Iraqi waters.
"Had we resisted, there would have been a major fight, one which we could not have won, and with consequences that would have a major strategic impact," said Captain Chris Air.
"We were 1.7 nautical miles from Iranian waters," Lieutenant Felix Carman told the news conference.
Britain has suspended boarding operations in the Gulf and is reviewing rules of engagement in the area's waters amid disquiet over how easily the sailors were seized on March 23.
"As part of this ongoing review, the operational procedures and the rules of engagement that go with them will be reconsidered," navy chief Admiral Jonathon Band told BBC Radio.