Four prisoners were killed and 61 others injured after a fire broke out at the notorious Evin prison in Tehran, Iran’s state news agency said.
The blaze erupted late on Saturday at Evin prison, which holds political prisoners and dual-national detainees. Gunshots were also heard.
Authorities said the fire was extinguished hours after the incident and that no prisoners escaped.
State media said it broke out after a fight between prisoners, in an apparent attempt to distance the events there from the continuing protests following the death of Iranian-Kurdish woman Mahsa Amini in the custody of the "morality police".
Hundreds of inmates are held at Evin, where human rights groups have reported repeated abuses of prisoners.
State TV on Sunday aired a video of the fire’s aftermath, showing scorched walls and ceilings in a room it said was the upper floor of a sewing workshop at the prison.
“This fire was caused by a fight between some prisoners in a sewing workshop,” said Tehran Governor Mohsen Mansouri. “The workshop was set up to create jobs” for prisoners.
"The remarks of the American president, who is inciting chaos, terror and the destruction of another country, serve as a reminder of the eternal words of the founder of the Islamic republic, who called America the Great Satan," Mr Raisi said, referring to the late Ayatollah Ruhollah Khomeini.
"The enemy's plot must be countered by effective measures to resolve people's problems."
Iranian social media posts challenged state media claims over the cause of the fire and apparent explosions at the prison.
Former inmate of Evin and rights activist Atena Daemi said in a tweet that normally all prisoners were sent to their wards and the workshops closed by sunset.
Families of inmates gathered on Sunday near the prison hoping for news of their loved ones inside.
Masoumeh, 49, who only gave her first name, told AP that her son, 19, was taken to the prison two weeks ago after taking part in the street protests.
“I cannot trust news about his health, I need to see him closely,” she said.
Reza, who also gave only his first name, said his brother had been in Evin Prison since last year after he was involved in a violent quarrel.
“He did not call us in recent days and following last night’s fire I am here to learn what happened to him,” Reza said.
US State Department spokesman Ned Price said Washington was following the reports from Evin prison "with urgency".
“We are in contact with the Swiss as our protecting power," Mr Price said. "Iran is fully responsible for the safety of our wrongfully detained citizens, who should be released immediately."
The EU’s top diplomat Josep Borrell expressed his “most serious concern” to Iran’s Foreign Minister, Hossein Amirabdollahian and called for “maximum transparency on the situation” after the prison blaze and apparent violence.
Iranian authorities are responsible for the lives of “all detainees, including human rights defenders and EU nationals". Mr Borrell said in a tweet on Sunday.
Siamak Namazi, an Iranian American imprisoned in Iran for nearly seven years on espionage-related charges rejected by Washington as baseless, returned to Evin on Wednesday after being granted a brief furlough, his lawyer said.
"Siamak Namazi has now spoken to his family. He is safe and has been moved to a secure area of Evin prison. We have no further details," Jared Genser said in a tweet.
The prison has been charged by rights groups with abusing inmates. It has long been known for holding political prisoners and those with ties to the West who have been used by Iran as bargaining chips in international negotiations.
The prison fire occurred as protesters intensified anti-government demonstrations along main streets and at universities in the capital and other cities across Iran on Saturday.
Human rights monitors reported hundreds dead, including children, as the movement concluded its fourth week.
Wider protests in the northern city of Ardebil erupted after reports a teenager, Asra Panahi, died after police confronted protesting girls at a high school.
Officials denied the report, saying she died because of a chronic heart problem and police did not hit her.
The protests erupted after public outrage over the death of Amini. She was arrested by Iran’s morality police in Tehran for breaching the strict dress code.
Iran’s government insists Amini was not mistreated in police custody, but her family says her body showed bruises and other signs of beating after she was detained.
Mr Biden, on a trip to Oregon, said the Iranian “government is so oppressive” and that he had an “enormous amount of respect for people marching in the streets".
Earlier Sunday, Iran's foreign affairs spokesman Nasser Kanani shrugged off Mr Biden's comments, saying: "Iran is too strong for its will to be swayed by the interference … by a politician tired of years of failure."
"We will together defend the independence of Iran," Mr Kanani wrote on Instagram.