Iran’s crackdown on journalists and political activists is being spearheaded by a small group of judges under the influence of the country’s intelligence and security apparatus, according to human rights organisations.

Four judges with Iran’s revolutionary court and two appeal judges have led numerous court sessions that activists say did not conform to fair trial principles according to Iran’s constitution, and are in breach of international treaties to which Tehran is a signatory.

The six judges are accused of losing their judicial impartiality and overseeing miscarriages of justice in trials in which scores of journalists, lawyers, political activists and members of Iran’s ethnic and religious minorities have been condemned to lengthy prison terms, lashes and even execution.

Those accused are judges Abolghassem Salavati and Mohammad Moghiseh, former justices Yahya Pirabbasi and Hassan Zareh Dehnavi (known as judge Haddad), and appeal judges Hassan Babaee and Ahmad Zargar.

According to several former prisoners who spoke to the Guardian, and testimonies received by human rights groups, common violations by the judges include holding trials behind closed doors, lasting only a few minutes and without essential legal procedures, intimidating defendants, breaching judicial independence by acting as prosecutors themselves and depriving prisoners of access to lawyers.

“This group is among the most notorious judges in Iran,” said Mahmood Amiry-Moghaddam, an Iranian human rights activist in Norway. “They are known for their politicised verdicts, unfair trials [and] sentencing prisoners based on confessions made under duress.”

Gissou Nia, of the US-based Iran Human Rights Documentation Centre (IHRDC), said: “It seems that in the courtrooms of Salavati, Moghiseh and Pirabbasi, there is [something] counter-intuitive at play – that is, the shorter the hearing, the longer the sentence.”

She added: “Those cases that have made their way before this trio of revolutionary court judges, and have resulted in long terms of imprisonment or, even worse, death, read like a who’s who of the most high-profile miscarriages of justice in the Iranian legal system.”

Islamic revolutionary courts were set up following the 1979 Iranian revolution to deal with cases of national security, and still exist three decades on despite disputes over their constitutional footing. Iran’s judiciary, which is at odds with President Hassan Rouhani’s government, has stepped up its crackdown on journalists in recent months.

The Washington Post’s correspondent in Tehran, Jason Rezaian, and his wife, Yeganeh Salehi, are the latest victims of the crackdown. It is not clear whether any of the six judges are presiding over Rezaian’s case.

In their testimonies, many prisoners have accused the six judges of acting on the instructions of top security officials and prison interrogators, and collaborating with the country’s intelligence ministry or the elite Revolutionary Guards.

Several prisoners said the sort of sentences they were threatened with in interrogation sessions were later handed down in their trials, which they say points to close collaboration between judges and the intelligence apparatus.

“While it is true that all the judges in the Iranian system are filtered and selected, it would be true to say that specific judges of revolutionary courts are more trusted with sensitive cases,” said the Iranian human rights lawyer Mohammad Nayyeri. “Their 100% loyalty is proven beyond any doubt and higher judicial authorities carefully select them on the basis of absolute loyalty and obedience. They work hand in hand with the intelligence service officers and to a great extent follow the instructions from them.”

According to Amiry-Moghaddam, judge Salavati has handed down at least half a dozen death sentences since 2009. In that year, Salavati led an infamous televised group court at which political activists were tried.

Amiry-Moghaddam said Babaei presided over the trial of the Kurdish political prisoner Habibollah Latifi, who was sentenced to death in 2010 after a trial that lasted a few minutes. Moghiseh led a court case against seven leaders of the Baha’i faith, who were kept for more than two years in solitary confinement without access to lawyers and later sentenced to 20 years each in jail.

Moghiseh also sentenced the prominent student activist Bahareh Hedayat to nine years in jail, and is currently presiding over the case of imprisoned journalist Saba Azarpeik. Former prisoners told IHRDC that Moghiseh was “notorious for creating an atmosphere of rage and tension” in court.

Pirabbasi has a notorious track record in condemning human rights lawyers, student activists, journalists, Christians and Baha’is. Under Pirabbasi, the celebrated human rights lawyer Abdolfattah Soltani was sentenced to 18 years, the student activist Zia Nabavi to 15 years and 74 lashes, Hossein Ronaghi-Maleki to 15 years and the award-winning lawyer Nasrin Sotoudeh to six years. Gholamreza Khosravi, who was put to death for allegedly having links with the opposition group MKO, was sentenced by Pirabbasi.

The six judges could not be reached for comment.

Karim Lahidji, president of the League for the Defence of Human Rights in Iran, said the abuses were reminiscent of similar unfair trials held before 1979 under the late shah and his Pahlavi dynasty. “Under the shah, such trials were held in military courts, now they’re held in revolutionary courts,” he told the Guardian.

According to Lahidji, under Iranian law convicts should serve only the longest sentence they have been given and not the total of their sentences for various charges, as is the current routine in Iran.

Bahareh Davis, an Iran researcher with Amnesty, said the Iranian revolutionary courts often defied not only international but also domestic standards on the independence and impartiality required for a just judicial system.

“Security bodies are frequently reported to interfere in judicial cases,” Davis said. “The revolutionary courts generally dismiss allegations of torture made by detainees who, in many instances, are held incommunicado for days, weeks or even months, and sometimes convict individuals in hearings that are only a few minutes long.”

Faraz Sanei of Human Rights Watch said Iran’s judiciary and its revolutionary courts had been among the main perpetrators of rights violations in Iran. “Instead of acting as an independent check on the abusive actions of security and intelligence forces, they have frequently either looked the other way or aided and abetted these forces in perpetrating egregious rights violations,” he said.

“The list of abuses is extensive. It includes long periods of solitary confinement, preventing detainees from meeting with their lawyers and family members or mounting a proper defence, torture and ill-treatment, televising coerced confessions, and issuing long and harsh prison sentences for peaceful activism.”

Human rights activists have also pointed fingers at other judiciary officials in Iran in recent years, accusing them of rights violations, including the appeal judge Movahed, the Tehran prosecutor Abbas Jafari Dolatabadi and his predecessor Saeed Mortazavi.