Eight Iranian inmates were killed in a fire that raged through Tehran's notorious Evin prison, the judiciary has said, doubling the official toll from the blaze that further stoked tensions one month into protests sparked by the death of Mahsa Amini.

Authorities in the Islamic republic have blamed the fire late Saturday on "riots and clashes" among prisoners, but human rights groups said they doubted the official version of events and also feared the real toll could be even higher.

The judiciary authority's website Mizan Online said that four Evin prison inmates injured in the fire had died in hospital, after reporting the previous day an initial toll of four dead from smoke inhalation.

Gunshots and explosions were heard during the dramatic blaze from inside the complex as flames lit up the night sky and smoke billowed from the building, in video footage posted on social media channels.

The Iranian authorities have accused "thugs" of torching a prison clothing depot and reported clashes between prisoners, and then between inmates and guards who intervened to put an end to the violence.

Hundreds of the protesters arrested in recent weeks have been sent to Evin, infamous for the ill-treatment of political prisoners, which also holds foreign detainees and thousands jailed on criminal charges.

The official IRNA news agency, citing a Tehran prosecutor, said the clashes had "nothing to do with the recent unrest in the country", while Mizan said that all those who died had been convicted of robbery.

But Oslo-based group Iran Human Rights (IHR) said it "rejects" the official account, given the "long history of concealing facts" in the Islamic republic.

It said it had "received reports that special forces were deployed to incite prisoners and set the grounds for a crackdown" and called for a UN-backed international investigation to establish the facts.

Prisoners' relatives and rights groups have voiced grave fears for the inmates and said Iranian security forces had used tear gas inside the correctional facility.

'He can barely speak'

The fire came after four weeks of protests over the death of 22-year-old Amini, following her arrest for allegedly violating Iran's strict dress code for women.

The wave of demonstrations has turned into a major anti-government movement, confronting Iran's clerical leadership with one of its biggest challenges since the ousting of the shah in 1979.

More protests were held yesterday, including at the Tehran and Shariati universities where women chanted "we are all Mahsa!", followed by more overnight rallies in some other areas of the capital.

Iranian rights activist Atena Daemi, herself a long-time inmate of Evin, wrote on Twitter that in the early hours of yesterday several buses and ambulances were seen leaving the facility.

She said some prisoners in Ward 8, which houses political detainees, had been transferred to another jail.

IHR reported that inmates' relatives gathered outside Evin yesterday, seeking information about their loved ones.

Activists noted further confusion when state television announced yesterday that 40 people had been killed, only to correct this back to the initial toll of four just minutes later.

Evin prison holds French-Iranian academic Fariba Adelkhah and US citizen Siamak Namazi, whose family said he was taken back into custody days ago after a temporary release.

Namazi's US attorney Jared Genser said he had spoken to his family, and that he was unharmed.

France said it was following "with the greatest attention" the situation of its citizens "arbitrarily detained" in Evin.

Supporters of Austrian prisoner Massud Mossaheb said he was suffering after inhaling smoke and tear gas, writing on Twitter that "he can barely speak... He is in big distress".

EU agrees to level new sanctions

European Union foreign policy chief Josep Borrell said the bloc expected "maximum transparency on the situation" at Evin.

The EU has agreed to level new sanctions, a move expected to be endorsed by its foreign ministers Monday.

At least 122 people have been killed in Iran's crackdown on more than a month of nationwide protests sparked by the death of Mahsa Amini, the Oslo-based group Iran Human Rights has said.

At least another 93 people were also killed during during separate clashes in the city of Zahedan, in the southeastern province of Sistan-Baluchistan, IHR said in a statement.

The group, updating an earlier toll of 108 dead, said that among those killed nationwide were 27 children.

Iranian President Ebrahim Raisi yesterday accused US President Joe Biden of "inciting chaos" after Mr Biden expressed support for the protests, while the head of the Revolutionary Guards accused the West of a cultural "invasion" of Iranian schools.

"The riots are a path that has come from strategic think tanks in America and England which has spread to our classrooms," the Guards' Sepah News website quoted Major General Hossein Salami as saying.

The crackdown has seen the arrest of hundreds of ordinary protesters and also dozens of civil society figures, including journalists, filmmakers and even athletes.

Prominent Iranian lawyer Saeid Dehghan wrote on Twitter that a total of 19 lawyers who had been working to defend those arrested had themselves been detained.