Sunday Timesjournalist Mr Peter Hounam has been released a day after he was arrested over contacts with nuclear whistleblower Mordechai Vanunu.
"They accused me of spying on nuclear secrets and aggravated espionage. It is laughable," Mr Hounam, who was arrested on Wednesday by Israel, told reporters as he walked out of a Jerusalem detention centre on Thursday.
He said he was held in a "dungeon with excrement on the walls."
Mr Hounam, who broke Vanunu's account of Israeli atomic secrets in 1986 in the Sunday Times, was arrested in connection with an as yet unbroadcast television interview that Israel said he arranged with the former nuclear technician.
Security officials, briefing foreign reporters about an arrest some Israeli politicians described as having portrayed Israel as an opponent of a free press, said they had wanted to see if Mr Hounan had classified material in his possession.
Israel has prohibited Vanunu from travelling overseas after he was released from jail in April following completion of an 18-year prison term for treason.
Mr Hounam challenged Israeli statements that Vanunu could have more secrets to spill. "I know it's not true because I did the original story and I made sure when I did it that everything he knew of any value was taken down in detail at the time," he said.
"The key fight is the fight to get Mordechai Vanunu the right to leave this country, start a new life in America, if that's where he wants to go, and stop these ridiculous restrictions," he said.