A 37-year-old man has been executed in Iran after being found guilty of heresy and insulting prophet Jonah, according to human rights activists.

Mohsen Amir-Aslani was arrested nine years ago for his activities which the authorities deemed were heretical. He was engaged in psychotherapy but also led sessions reading and reciting the Qur’an and providing his own interpretations of the Islamic holy book, his family said.

Amir-Aslani was hanged last week for making “innovations in the religion” and “spreading corruption on earth”, but human rights activists said he was a prisoner of conscience who was put to death because of his religious beliefs. He had interpreted Jonah’s story in the Qur’an as a symbolic tale.

Iran’s judiciary, which was responsible for the handling of his case, has since denied that Amir-Aslani’s execution was linked to his religious beliefs.

Instead, the authorities allege that he had illicit sexual relationships with a number of people who participated in his sessions and the type of activities he was involved in did not follow an official interpretation of the religion. It was not clear if Amir-Aslani had official permission to conduct his sessions.

“Mohsen held sessions in his own house dedicated to reciting the Qur’an and interpreting it. He had his own understandings [of the religion] and had published his views in the form of a booklet and made it available to his fans,” an unnamed source told the New York-based group, the International Campaign for Human Rights in Iran (ICHRI).

According to the source, Iran’s ministry of intelligence was behind Amir-Aslani’s arrest. “He was initially held for making innovations in Islam and providing his own interpretations of the Qur’an but later he was accused of insulting prophet Jonah and also faced accusations of having sex outside marriage,” the source said. “They alleged that he had sexual relationships with a group of the people who participated in his classes.”

Iran’s judiciary has presented little evidence in public relating to the allegations of illicit sexual activities. The judge who presided over his case, Abolghassem Salavati, is known in Iran for leading numerous unfair trials, including many that resulted in execution.

Amir-Aslani’s wife, Leila, told the opposition website Roozonline that she was hoping a high court would strike down his conviction but his sentence was eventually upheld. She told Roozonline that his conviction stemmed from his religious views and no evidence was presented to back up the charges related to his alleged sexual activities.

Iranian authorities are sensitive towards those practising Islam in ways not conforming to the official line. In recent years, several members of Iran’s Gonabadi dervishes religious minority have been arrested and are currently serving lengthy prison terms.

Amnesty said last week that a group of nine Gonabadi dervishes were on hunger strike in protest at their treatment in prison. They were Mostafa Abdi, Reza Entesari, Hamidreza Moradi and Kasra Nouri, as well as the five lawyers representing them who have also been jailed: Amir Eslami, Farshid Yadollahi, Mostafa Daneshjoo, Afshin Karampour and Omid Behrouzi.

“The men were mostly detained in September 2011, during a wave of arrests of Gonabadi dervishes. They were all held in prolonged solitary confinement, without access to their lawyers and families, and were sentenced, after two years and following grossly unfair trials, to jail on various trumped-up charges,” Amnesty said. “The men are prisoners of conscience, imprisoned solely for practising their faith and defending the human rights of dervishes through their legitimate activities as journalists and lawyers.”

In Iran, Gonabadi dervishes face persecution, discrimination, harassment, arbitrary arrests and attacks on their prayer houses, Amnesty said.

• The caption with this article was amended on 2 October 2014 to make clear that the picture is a campaign poster.