Norway-based Iran Human Rights said 26 men were executed in Ghezelhesar Prison in Karaj outside Tehran, while three other men were executed in Karaj's city prison.
Those executed, who included two Afghan nationals, had been convicted of murder, drug-related and rape charges.
Other rights groups including US-based Human Rights Activists News Agency (HRANA) and Center for Human Rights in Iran (CHRI) also confirmed the execution of at least two dozen people in Karaj.
Human rights groups have repeatedly accused Iran, which they say executes more people annually than any nation other than China, of making use of the death penalty on all charges to instill fear in society in the wake of the 2022 protests.
"Without an immediate response from the international community, hundreds of individuals could become victims of the Islamic republic's killing machine in the coming months," said IHR director Mahmood Amiry-Moghaddam.
IHR emphasised that a group execution on this scale was unprecedented in recent years in Iran, with the last comparable example dating back to 2009.
'Abhorrent arbitrary execution'
Human rights groups also condemned Iran's execution of a man convicted of killing a Revolutionary Guard in 2022 protests, with activists saying his confession had been obtained by torture.
Gholamreza Rasaei, in his mid-thirties, was the 10th man executed by Iran in connection with the months-long protests that erupted in September 2022 after the death in custody of Mahsa Amini.
Amini, a 22-year-old Iranian Kurd, had been arrested for an alleged breach of the country's strict dress code for women.
Rasaei was hanged in prison in the western city of Kermanshah on Tuesday after being convicted of killing the Guards colonel, according to the Mizan Online website of the Iranian judiciary.
Rasaei, a member of the Kurdish ethnic minority and follower of the Yarsan faith, was executed in secret with neither his family nor his lawyer given prior notice and his family then forced to bury his body in a remote area far from his home, Amnesty International said.
"Iranian authorities have carried out the abhorrent arbitrary execution in secret of a young man who was subjected to torture and other ill-treatment in detention... and then sentenced to death after a sham trial," said Amnesty's deputy director for the Middle East and North Africa, Diana Eltahawy.
She said the execution was another instance of Iran using the death penalty as a "tool of political repression to instil fear among the population".
Amnesty said his death sentence was handed out in October 2023 "after a grossly unfair trial that relied on his forced 'confessions' obtained under torture and other ill-treatment, including beatings, electric shocks, suffocation and sexual violence".
'Unfair and inhumane'
France's foreign ministry on Wednesday condemned Rasaei's execution, and reiterated its "unchanging opposition to the death penalty in all places and circumstances", calling it an "unfair and inhumane punishment".
The office of the US deputy special envoy for Iran Abram Paley accused Iran of subjecting protesters to "sham trials and forced confessions".
IHR said Rasaei had stated in court that the confessions had been obtained under torture, but this was ignored by the judge who also dismissed two expert testimonies, including a forensics report, that argued he could not have been behind the killing.
IHR said Iran has now executed at least 345 people this year alone, adding the latest executions showed there was no let-up in its use of the death penalty since reformist President Masoud Pezeshkian was sworn in last week.
Amiry-Moghaddam said that Iran was "exploiting" global focus on the tensions between Iran and arch-enemy Israel by "mass killing prisoners and intensifying the suppression in Iran."
