Many expressed anger over the death in custody of Kavous Seyed Emami, 63, a Canadian-Iranian citizen and respected professor at Imam Sadegh University who was the founder of the Persian Wildlife Heritage Foundation.
Canada said it has concerns about the case and asked Tehran for answers.
Meanwhile, the deputy head of Iran's Environmental Protection Organisation, Kaveh Madani, appointed by President Hassan Rouhani last year, released a video that appeared to support claims that he too had been temporarily detained in recent days.
Emami's death prompted Iranian rights campaigner Emadeddin Baghi, who has been jailed several times, to express regret over his failure to speak up about prison abuses in the past.
"When I heard this news, I felt guilty because, in order to prevent it being exploited by Iran's enemies... I refused to reveal the bad treatment I had experienced during my detention," Baghi wrote on his Telegram channel.
"If we had all spoken out, it would be known why such catastrophes happen in prisons."
Emami's family was told Friday that he had killed himself in prison two weeks after his arrest.
"Canada is concerned about the circumstances around the death of Mr Seyed Emami," said Omar Alghabra, parliamentary secretary to the minister of foreign affairs, adding that Ottawa "has asked Iranian authorities for answers."
A judiciary official claimed Sunday that Emami had confessed to crimes related to an espionage investigation, which has seen seven other members of his wildlife NGO placed in detention.
A group of four academic societies, representing some of Iran's top universities, wrote an open letter to Rouhani, demanding "immediate and effective action to seriously investigate the case... and make the institutions involved in this painful loss accountable".
"In addition to being a well-known professor, a distinguished scientist and war veteran... he was a noble and ethical human being," they wrote.
"The news and rumours related to his arrest and death are not believable."
One of Rouhani's closest advisors, Hesameddin Ashena, tweeted later that the judiciary, which is dominated by conservatives and has clashed with his moderate government in the past, should be more closely supervised.
"Judges, prosecutors, officers, interrogators are neither infallible, nor faultless or free from prejudice," Ashena wrote.
"Just as it is necessary to supervise the executive branch, it is necessary to supervise their dealings with defendants."
Asked about Emami's case, judiciary spokesman Gholamhossein Mohseni Ejeie told the reformist ILNA news agency: "I have heard he committed suicide but I have so far no information on the details. This recent incident is under investigation."
'I am safe'
Meanwhile, questions continued to circulate about Madani, the 36-year-old deputy head of the Environmental Protection Organisation (EPO), a well-known water conservation activist who was appointed amid much fanfare by the government in September.
A reformist lawmaker tweeted that Madani had been arrested over the weekend, which was later denied by officials at the EPO.
Madani released a cryptic video on Instagram on Monday in which he suggested he had faced problems.
"Thanks to all my friends and loved ones who followed up on my situation and I hope no one will face any trouble. I wanted to say that I am safe," he said.
Madani added that his access to email, Twitter and Telegram had been cut, but "hopefully this will be resolved."
"These issues will pass and, God willing, narrow-mindedness will be eliminated and we can develop our country in a sustainable way... and secure our environment for the next generation."
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