Protests have been ongoing in Iran after the death of a 22-year-old woman in police custody. Photo: AFP
A fire has broken out at Iran's notorious Evin prison, with footage posted online showing flames and smoke billowing from the area.
Gun shots, explosions and alarms have been reported as coming from the jail, the primary site for detaining political prisoners.
An official quoted by state media said "criminal elements" were to blame, but that the situation was under control.
Iran has been gripped for weeks by anti-government protests.
They first erupted last month when anger over the death in police custody of 22-year-old Kurdish Iranian Mahsa Amini boiled over. Officials say she died from an underlying health condition, but her family say she died after being beaten by morality police.
BBC Persian's Rana Rahimpour said they do not yet know if the situation at the prison is linked to the recent demonstrations, but it easily could be because hundreds of the protesters have been sent to Evin.
Inmates there will be fully aware of how intense the situation is in the country, Rahimpour reports.
Mahsa Amini. Photo: Supplied
Anti-government monitoring group 1500tasvir posted videos of the fire online, in one of which chants of "death to the dictator" - one of the main slogans of the anti-government protest movement - could be heard in the background.
Another video posted on social media appears to show objects being fired into the prison from outside its perimeter and an explosion is then heard, according to BBC Persian.
Iranian media, which is strictly controlled by the government, reported that the riot started with financial and criminal prisoners and that no political prisoners were involved. The fire broke out in a prison workshop but is being put out, and the rioters have been separated from other prisoners, it said.
It added that no deaths had been reported and eight people were injured. The BBC was unable to verify these figures.
The families of some prisoners have told BBC Persian that they are unable to get through to their relatives on the phone, and that internet connection around the prison also appears to have been cut off.
BBC Persian also says that the streets leading to the prison are closed.
Earlier, videos showed riot police entering Evin and there are reports that special forces have been deployed to the area. A witness told Reuters news agency that ambulances were also there.
As well as political prisoners, journalists and many dual and foreign nationals are also held at the prison.
British-Iranian dual nationals Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori were both held on spying charges, which they denied, before their release earlier this year. They were imprisoned for six and five years respectively.
The prison has long been criticised by Western rights groups. Human Rights Watch has accused authorities at the prison of using threats of torture and of indefinite imprisonment, as well as lengthy interrogations and denial of medical care for detainees.
A group of hackers calling themselves Edalat-e Ali (Ali's Justice) posted videos in August last year of leaked surveillance footage from Evin prison showing guards beating or mistreating inmates.
Since the death of Amini five weeks ago a wave of protests have swept across Iran in the boldest challenge to the Islamic Republic since its establishment, in 1979.
Iran's Human Rights Activists News Agency (HRANA) estimates at least 222 people have been killed during the widespread demonstrations sparked by the death in custody of Amini, detained for allegedly breaking rules on headscarves.
Many of those who have died have been teenagers.
- BBC