Mostafa Tajzadeh (a prominent reformist politician) was arrested in Iran and charged with conspiring against Iran's security.
He is also accused "publishing false information intended to disturb public opinion". He was a former deputy interior minister and spent seven years in prison between 2009 and 2016.
He is a strong critic of Supreme Leader Ayatollah Muhammad Khamenei.
For a separate protest, two Iranian film directors were also arrested.
Mohammad Rasoulof has won awards at the Berlin and Cannes film festivals for his films. Both he and Mostafa alahmad are being held for inciting unrest.
They were part of an Iranian cinema group that protested the May collapse of a 10-storey Abadan building, which resulted in more than 40 deaths.
In an open letter, they urged security forces to stop using force and allow protests to continue after the Metropol office block collapse.
Sebastian Usher, BBC Middle East Editor, says that the Abadan incident became a rallying cry against the authorities. Protests were held not only there, but also elsewhere in Iran as protestors blamed the government for their negligence and corruption.
Former President Mohammad Khatami was Mr Tajzadeh's colleague. He has criticised President Ebrahim Rashi and the clergy.
He has been a long-standing advocate for democratic change in Iran.
Fars news agency reported that he had tweeted hours before his arrest against the government's tightening of Islamic dress codes for women.
