The delay in starting Saeed Mortazavi's two-year sentence had caused anger and ridicule in recent weeks, with mocked-up "Wanted" posters appearing for the notorious former prosecutor online and around Tehran.
Mortazavi's lawyer was unable to confirm whether he had been taken into custody, telling the reformist ILNA news agency: "I have no information on this issue."
But the judiciary-linked Mizanonline news agency, citing an unnamed source, said: "The process of transferring Mortazavi to prison to serve his term is underway."
He "was identified and arrested some hours ago by law enforcement officers in one of the northern cities of the country," it added.
The former chief prosecutor for Tehran was sentenced last year over the death of a protester who had been jailed for taking part in the mass protests over alleged election-rigging in 2009.
In recent days, reformist lawmakers had demanded answers over why Mortazavi was still a free man, particularly after a judiciary spokesman said the authorities had "not been successful in getting (their) hands on him".
A journalist for the Ghanoon newspaper, Mira Ghorbanifar, tweeted earlier this month that she had phoned Mortazavi and he had calmly told her he was at home in Tehran.
Mortazavi is subject to sanctions by the United States and the European Union for "grave violations of human rights" including the arrest of "hundreds of activists, journalists and students" during the 2009 protests.
He was suspended from the judiciary in November 2014 in connection with the deaths of three protestors in Kahrizak prison, south of Tehran, in July 2009.
Mortazavi was ultimately found guilty in relation to one of the three deaths and sentenced to five years, but this was commuted to two years in November, with no possibility of further appeal.
He had already become a hate figure among reformists in the early 2000s, when he earned the nickname "the butcher of the press" for closing dozens of reformist newspapers and cracking down on dissident bloggers.
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