The Amnesty International human rights group says those detained in recent days include Iranian ethnic Azeris advocating a school boycott and at least 10 people who sought to demonstrate against the imminent execution of four women.
Amnesty is calling on the Iranian authorities to cease harassment of those who peacefully exercise their rights.
The group highlighted the case of prominent human rights defender Ali-Akbar Musavi-Khoeni, who has been detained without charge or trial for more than three months.
Khoeni, a former reformist legislator, disclosed during a short prison leave that he has been tortured in detention in a bid to make him "repent."
Human Rights Watch has also expressed grave concern over Khoeni's health and safety and called for his release.West Accused Of Recruiting Spies
Meanwhile, a statement by the International Federation of Journalists (IFJ) has referred to comments made last week by Iranian Intelligence and Security Minister Gholam-Hussein Mohseni-Ejei, who claimed that the country's intelligence service has found that the West was recruiting journalists, students, and workers for spying overseas under the guise of research or study.
The Association of Iranian Journalists has told the IJF that the statement will lead to more pressure on journalists as any foreign trips could make them targets of accusation of spying.
Christopher Warren, president of the Brussels-based IFJ, says the pressure from Iran's government is particularly worrying in light of recent closures of newspapers and increased crackdown on media.