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

Authorities in the Islamic Republic have blamed the fire late on 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 on Monday 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.

'Big distress'

The fire came after four weeks of protests over the death of 22-year-old Kurdish Iranian woman Amini, following her arrest for allegedly wearing her hijab 'incorrectly'. Amini's Kurdish name can be spelt "Jina" or "Zhina".

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 on Sunday, 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 Sunday 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 on Sunday, seeking information about their loved ones.

Activists noted further confusion when state television announced on Sunday 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".

'Inciting chaos'

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

Iranian authorities blamed "troubles" between inmates and guards for the fire that broke out at Tehran's Evin prison 👇

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

At least 108 people have been killed by security forces in the crackdown on the Amini protests, and at least 93 more died in separate clashes in Zahedan, Sistan-Baluchestan province, according to IHR.

Iranian President on Sunday accused US President Joe Biden of "inciting chaos" after 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.