No details on the charges against Mahmoudian were available.
The arrest came just days after Mahmoudian and 16 others signed a statement condemning Islamic Republic leader Ayatollah Ali Khamenei and the regime's violent crackdown on demonstrators.
Two other signatories, Vida Rabbani and Abdullah Momeni, were also arrested.
Jafar Panahi, the prize-winning director of It Was Just an Accident, issued a statement decrying his co-writer's arrest.
"Mehdi Mahmoudian is not just a human-rights activist and a prisoner of conscience; he is a witness, a listener, and a rare moral presence - a presence whose absence is immediately felt, both inside prison walls and beyond them," Panahi said.
Panahi was also a signatory on the January 28 statement. It reads in part: "The mass and systematic killing of citizens who bravely took to the streets to bring an end to an illegitimate regime constitutes an organised state crime against humanity."
It Was Just an Accident is nominated for best screenplay and best international film at the March 15 Oscars. The film, made covertly in Iran, was France's nominee for best international film.
Panahi, one of the most acclaimed international filmmakers, has made films through various states of imprisonment, house arrest and travel ban.
It Was Just an Accident, a revenge drama and the Palme d'Or-winner at last year's Cannes Film Festival, was inspired by Panahi's most recent stint in prison. It was there that he met Mahmoudian.
Panahi called him "a pillar" to other prisoners.
Last year, Panahi was again sentenced to a year in prison and given a two-year ban on leaving Iran after being convicted on charges of "propaganda activities against the system."
The director, who has been traveling internationally with the film, has said he will return to Iran despite the sentence.
The US-based Human Rights Activists New Agency, which relies on a network inside Iran to verify its information, says that more than 6,713 people have been killed and 49,500 people have been detained in the recent Iranian government crackdown.
The Associated Press has been unable to independently assess the death toll and arrest figures.
Panahi has repeatedly spoken out against the crackdown.
"As we stand here, the state of Iran is gunning down protesters and a savage massacre continues blatantly on the streets of Iran," Panahi said last month at the National Board of Review Awards in New York. "Today the real scene is not on screens but on the streets of Iran. The Islamic Republic has caused a bloodbath to delay its collapse."
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