But evangelical preacher Eddie Romero said he was treated well after being arrested Monday and held for over 24 hours in the Iranian capital before being put on a plane out of the country.
"We had long good discussions. The primary interrogator spoke very good English. We really had a serious, serious discussion," he told reporters at Los Angeles airport.
"To me it was just a wonderful time together," he said.
The US pastor staged his protest outside Tehran's Evin prison, calling for the release of five inmates who he said were prisoners of faith and conscience, including an Iranian Christian pastor and a prominent human rights lawyer.
Romero managed to livestream about five minutes of his protest, before being arrested and taken into the prison for questioning, according to organizers, who said he was detained for 32 hours.
He had been traveling with a tour group but left it three days before the protest, and went into hiding. After being arrested and detained overnight, he was handed over to the Swiss ambassador in Tehran, he said.
"I'm a little bit unkempt right now, only because I have spent four nights on the streets of Tehran as a homeless person," said Romero, who is based in southern California, just outside Los Angeles.
"At least that's what I appeared to be, until I was able to go to the gates of the prison and raise my protest, and then be taken in," he added.
Explaining why he did it, he said: "It is for freedom that Jesus Christ has set us free, not for bondage, not for tyranny.
"It's a way of being able to put belligerent governments, those who do not choose to do human rights, to put them on notice."
Romero said he was strip searched twice and interrogated for about six hours, before spending the night in custody.
At one point he was taken before a Mullah. "He was a big stern man ... and he just started laughing, and he laughed because he couldn't believe I actually did that. That really set the tone for the rest of the time.
"These guys were trying to act tough, but they liked me. ..I was treated very well by them. There was never any time that they treated me brutally," he said.
He praised Iran's moderate President Hassan Rouhani, who has made huge efforts to engage with the West since succeeding his hardline predecessor Mahmoud Ahmadinejad.
"Rouhani is trying to set a new tone. There is some new hope in the air," said the pastor.