Tehran: Human rights activists in Iran have raised concerns about mass arrests during the country’s largest protests in nearly a decade after at least three demonstrators died in a notorious Tehran jail.

Two members of the Iranian parliament close to the reformist camp confirmed on Monday that one detainee, Sina Ghanbari, had died in Evin prison.

Separately, Nasrin Sotoudeh, a prominent human rights lawyer, told the Guardian on the phone from Tehran that at least two other protesters had died in the jail. They have not yet been identified.

“I spoke to a prisoner in Evin prison and I was told that three detainees had lost their lives,” Sotoudeh said. “When authorities resort to mass arrests, they cannot claim to protect their rights. It is not possible in such a situation for the judicial process to take its due course.”

Sotoudeh was particularly worried about the use of unofficial detention centres. During the protests of 2009 that followed Mahmoud Ahmadinejad’s disputed re-election as president, one such detention centre, Kahrizak, drew nationwide attention after it emerged that a number of protesters had been sexually assaulted, tortured and killed in custody .

Reformist politicians, including the outspoken MP Mahmoud Sadeghi, have warned of a repeat of the scandal. “I warn the president, intelligence and judicial officials against the repeat of a second Kahrizak,” he tweeted.

Sotoudeh has cast doubt over claims by officials that Ghanbari killed himself. She said his death showed a violent crackdown was indeed under way though it was too soon to know its scale. “In 2009, it took weeks before the scale of brutality at Kahrizak was laid bare,” Sotoudeh said. “The authorities are responsible for the health of prisoners.”

Mahnaz Afshar, a high-profile Iranian actor, tweeted: “There is no excuse for the death of a 23-year-old held in Evin.”

Sotoudeh said she was concerned about the prisoners’ legal representation at a time when many human rights lawyers were in jail, exiled or vilified.

Iranian authorities blame the protests on foreign enemies, but on Monday the president, Hassan Rouhani, alluded to a gap between the worldview of the authorities and the country’s youth, who make up most of the population. The average age of those arrested during the unrest was 25, officials said.

One protester being held at an undisclosed location is Kasra Nouri, a 26-year-old law student at Tehran University. He is a member of the Gonabadi dervish order, a minority Muslim sect not recognised by the Iranian authorities.

“He has embarked on a hunger strike since 4 January. We are completely kept in the dark about his situation in jail and his lawyers have not been able to gain access to him either,” a family member said.

Concern about the fate of thousands of detainees had been heightened by Ghanbari’s death, Hadi Ghaemi, the executive director of the Centre for Human Rights in Iran, said. “The authorities claim he [killed himself] but there is no credibility to their claims without an independent and fair investigation,” he added.

“Iranian people have fresh memories of 2009 detainees being raped and tortured systematically in detention centres such as Kahrizak and Evin. There are vocal calls in Iran to prevent the repeat of such atrocities.”

Nassim Papayianni, an Amnesty International researcher on Iran, said its investigations showed “time and time again just how inhumane prison conditions are in Iran, with overcrowding, poor ventilation and the ever-present threat of torture.

“We’re also concerned that the Iranian authorities are denying the family members of those arrested information about the fate and whereabouts of their loved ones. The authorities must end this wall of silence and provide family members information on those detained.”

Rouhani, re-elected in a landslide victory last year, is under pressure because the protests erupted soon after he announced an austerity budget.

Though the initial protest on 28 December was believed to have been organised by an anti-Rouhani group, the demonstrations grew as they spread across the country, transforming from economic to political protests challenging the foundations of the Islamic Republic.

Rouhani defended his economic performance and said it was “misleading and insulting” to suggest the protesters’ demands were limited to the issue. “People are right to say: ‘See us, listen to us, and respond to our demands’,” he said, calling for greater transparency.

“Everyone should go to the glass room so people can know and judge them. Authorities are not infallible and all authorities can be criticised.”

Acknowledging that younger people wanted change, Rouhani said: “One cannot impose their lifestyle on future generations.”

— Guardian News & Media Ltd, 2018