The jail is reported to be holding hundreds of people arrested during the month-long protests for human and civil rights. Several foreigners are also detained there.
"A fire is spreading in Ewin prison," said the Twitter channel 1500tasvir, which regularly reports on the protests and police violence in Iran. There was also an explosion.
The state news agency IRNA reported "riots and clashes" at the prison. "Riots" started an altercation with prison employees and set fire to the detention center's textile warehouse. However, the situation is "completely under control" again, and "calm has returned" to the detention center. According to the report, at least eight people were injured.
Iran has been rocked by violent protests for weeks. They were triggered by the death of the young Kurd Mahsa Amini. The 22-year-old died in Tehran on September 16 after being arrested by the vice squad three days earlier on charges of not wearing her headscarf in accordance with regulations.
Several foreign nationals are also being held in Ewin Prison. Responding to reports of the unrest and fire, the family of US citizen Siamak Namazi said they were "deeply concerned" and had not heard from Namazi. The sister of another US citizen detained in the prison wrote on Twitter that she was "sick with worry".
A US State Department spokesman said Washington was following the incident closely. Iran is "fully responsible for the security of our illegally detained citizens".
"Shots are being fired while Ewin burns," wrote researcher Roham Alvandi of the London School of Economics on Twitter. "Should political prisoners perish there, it will be an incident on the scale of the 'Cinema Rex' fire in Abadan in August 1978 which hastened the overthrow of the Shah."
Around 400 people died in an arson attack on the "Cinema Rex". The incident on the eve of the Iranian revolution sparked protests against the shah, even though the background to the attack was never clarified.
The protests in Iran continued on Saturday. At a demonstration at Shariati University in the capital Tehran, women without headscarves shouted slogans such as "The mullahs should go away!", according to a video distributed on the Internet. Other protests were reported from Isfahan and Kermanshah, among others.
According to 1500tasvir, young women at a college in Tehran shouted "Freedom, freedom, freedom" while waving their headscarves in the air. The Twitter channel also reported on striking shopkeepers in Kurdistan Province and West Azerbaijan.
Because of the violent crackdown on demonstrators in Iran, the EU countries had agreed on new sanctions against Tehran. According to diplomatic circles, the EU foreign ministers are to officially decide on the punitive measures at a meeting in Luxembourg on Monday.
