Iran has reportedly rearrested political activist and journalist Heshmatollah Tabarzadi, who was released last year after spending more than five years in jail for his outspoken criticism of the Iranian establishment.
His son said Tabarzadi had "very likely" been arrested on May 17 and reportedly was transferred to the special wing of a Tehran prison notorious for housing political prisoners for the country's intelligence service.
The reason for his arrest is unclear.
Iranian authorities routinely withhold information from relatives and the public about detainees and the charges against them, even once their trials have begun.
Abtin Tabarzadi wrote on social media that his father left the house in the morning and had not returned by evening.
He added later that he had received information that his father had been transferred to Section 209 of Evin prison, in the capital.
"[My father] had said repeatedly that in case of arrest, he will immediately go on an indefinite hunger strike," Tabarzadi wrote on Facebook.
Tabarzadi, the head of the banned Democratic Front of Iran, has been in and out of jail for the past two decades.
He was among the many activists and intellectuals arrested in the crackdown that followed the disputed reelection of Mahmud Ahmadinejad to the presidency in 2009. Tabarzadi was later sentenced to nine years in prison on charges that included "insulting Iran's leader" -- a reference to Supreme Leader Ayatollah Ali Khamenei -- and "spreading propaganda against the [Iranian] establishment."
Since his release from prison in July, he had continued to criticize the Iranian authorities while accusing them of committing human rights abuses and staging unfair elections.
In an act of defiance, Tabarzadi registered as a candidate for Iran's February parliamentary elections. As expected, his candidacy was not approved by the powerful Guardians Council that vets all of Iran's election candidates.
On May 11, Tabarzadi posted on Facebook pictures of a meeting with a former Baha'i cellmate who had been released from prison. In the post, Tabarzadi criticized the perceived persecution of Baha'is in the Islamic republic, where their faith is not officially recognized.
Several journalists have been arrested in Iran in recent months.
Earlier this week, Iranian news sites reported that the manager of a popular blogging service, Mehdi Butorabi, had been detained.
Iranian authorities also announced that they had arrested eight people involved in a modeling network on Instagram where pictures of female models were posted without the obligatory Islamic head covering.