(CNN) — Iran executed at least 582 people last year, a 75 percent increase on the previous year, according to human rights groups who say the rise reflects an effort by Tehran to "instill fear" among anti-regime protesters.

It was the highest number of executions in the Islamic republic since 2015, according to a report released Thursday by the Norway-based Iran Human Rights (IHR) and the France-based Together Against the Death Penalty (ECPM) groups.

The vast majority of the executions — at least 544 — were of people accused of murder and drug-related offenses, said the report. It added that almost 90 percent of the executions it recorded were not announced by Iranian authorities and some had been carried out in secret.

The two rights groups said the increase was Tehran's way of trying to frighten protesters and prevent dissent, following a nationwide uprising sparked by the death of 22-year-old Mahsa Amini last September.

"Iran's authorities demonstrated how crucial the death penalty is to instill societal fear in order to hold onto power," the report said.

Iranian authorities have responded to the protests with brute force, mass arrests and hasty sham trials, drawing sharp global condemnation and sanctions from the United States.

The report documented 15 executions carried out on the vaguely defined charges of "enmity against God" and "corruption on Earth."

Mohsen Shekari — reportedly the first person to be executed in connection with the protests — was hanged on December 8 after he was convicted of "waging war against God" for allegedly stabbing a member of the Basij paramilitary force, a wing of Iran's Revolutionary Guard, at a protest in Tehran on September 23. Less than a week later, Majidreza Rahnavard was also convicted for reportedly killing two members of the same paramilitary force and injuring four others on November 17.

Two other Iranian young men — Mohammad Mehdi Karami, a karate champion; and Seyed Mohammad Hosseini, a volunteer children's coach — were hanged on January 7 this year in connection with the protests, according to Iran's judiciary news agency Mizan. They were convicted of killing a member of the Basij paramilitary force in Karaj on November 3, Mizan reported. The human rights report said they were charged with "corruption on Earth."

Dozens of other protesters have received death sentences in recent months.

Iran's Supreme Leader Ayatollah Ali Khamenei has openly praised the Basij for its role in the crackdown, describing the protesters as "rioters" and "thugs" backed by foreign forces.

But United Nations High Commissioner for Human Rights Chief Volker Turk has criticized the crackdown as pushing Iran into a "full-fledged human rights crisis."

An 'execution machine'

More than half of the executions last year took place after the protests erupted in September. Some 44 percent of all those sentenced to death were accused of drug-related offenses, despite no evidence of a marked rise in drug use or trafficking reported by international agencies, the report said.

IHR Director Mahmood Amiry-Moghaddam suggested Iran would have executed even more people had it not been for "international reactions to the death sentences against protesters" which had "made it difficult for the Islamic republic to proceed" with the killings.

"To compensate, and in order to spread fear among people, the authorities have intensified executions for non-political charges. These are the low-cost victims of the Islamic republic's execution machine," Amiry-Moghaddam said.

"In order to stop this machine, the international community and civil society inside and outside Iran must show the same reaction to each and every execution," he added.

In the report, the two rights groups urged the international community "to increase efforts to support the demands of the Iranian people for respect of their fundamental human rights and the abolition of the death penalty."

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