As Tehran's prosecutor general, Saeed Mortazavi was also responsible for sending Iranian-Canadian photojournalist Zahra Kazemi to prison where she was tortured and killed.
The report by IRNA quotes an unnamed official as saying Mortazavi has been released.
He has been at the centre of an escalating confrontation between President Mahmoud Ahmadinejad and his conservative rivals in parliament ahead of the June presidential election.
An investigation by parliament two years ago found Mortazavi responsible for deaths by torture of at least three jailed anti-government protesters in 2009.
Kazemi was beaten, raped and killed after her arrest for photographing wives of detainees outside Tehran's notorious Evin prison.
Her son said Tuesday that he got no satisfaction from Mortazavi's reported arrest.
Stephan Hashemi told The Canadian Press that Mortazavi is merely a small fish in the country's repressive regime.
"It's the regime who are killing people, who are torturing people, who are creating all this misery," Hashemi said.
"It's not Mortazavi. Mortazavi is just one little guy among them.
"Who is Mortazavi? He's nobody."
Mortazavi, a close ally of President Mahmoud Ahmadinejad, had been taken to Tehran's Evin prison late Monday night, according to the reports, which gave few details on the arrest.
Evin is the notorious facility where Kazemi spent the last agonizing days of her life in 2003.
Hashemi and Kazemi's estate are suing the Iranian government in Canadian courts — a case that lists Mortazavi as one of those responsible for her death.
The suits are awaiting a decision by three judges from the Supreme Court of Canada as to whether they can proceed with an appeal process.
The lead lawyer for both Hashemi and the estate said Mortazavi's arrest would not impact the Canadian case.
"I'd be curious to know more about the reasons why Mr. Mortazavi's been arrested (and) what, if anything, he's charged with," attorney Kurt Johnson said Tuesday.
"Are there any charges or accusations made in connection with Ms. Kazemi?"
Johnson said the reported arrest came somewhat out of the blue.
"He seemed to have been a protected person within the regime for quite some time and I guess that's no longer the case," he said.
Canada has called Kazemi's killing a state-sanctioned murder.
Prime Minister Stephen Harper himself has demanded Mortazavi's arrest.
In 2006, Harper asked German authorities to apprehend Mortazavi if he stopped there on his way home from a human-rights conference in Geneva.
Though that German stopover didn't happen, Harper said Canada would not give up.
"We're appealing to the international community to use all manner of law available to detain this individual, and have him face justice," Harper said at the time.
Ahmadinejad cannot run a third time because of term limits and many of his allies have either been arrested or pushed to the political margins amid concern over the president attempting to extend his powers.
Ahmadinejad strongly denounced Mortazavi's arrest, accusing the judiciary of being run as a "family institution."
"This is very ugly. It's not appropriate for the people of the Islamic Republic and the judiciary," Ahmadinejad said before leaving Tuesday for Egypt, to attend a summit of the Organization of Islamic Cooperation.
"I'll definitely pursue this matter seriously when I return."
Mortazavi was Tehran city prosecutor in 2009, during massive protests in the wake of Ahmadinejad's disputed re-election.
At the time, he was responsible for Kahrizak prison in the Iranian capital, where at least three anti-government protesters were tortured to death. He now heads the country's social security fund.
Iran's reformists have openly criticized Mortazavi during his years in the judiciary.
He was even dubbed "butcher of the press" for the closures of more than 120 newspapers and the imprisonment of dozens of journalists and political activists over the past 13 years.
- with files from The Canadian PressSuggest a correction