According to Reporters Without Borders, Heidari was one of several participants in a religious ceremony in honor of political prisoners, held in the home of Shehabeldin Tabataba, one of the prisoners close to the reformist party.
Reports claim that some detainees were released the next day, but about a dozen, including Heidari, were transferred to Evin prison.
Heidari heads the website Persian Cartoons (www.haditoons.com) and was the cultural editor of the banned daily Etemad-e Melli. Editor of the newspaper, Mohammad Ghochani, a member of its editorial staff, Fayaz Zahed, and website editor, Mohammad Davari, are all still in prison.
Toonpool, a worldwide cartoonists' community joined by artists from 140 nations, sent a letter to the Iranian embacy in German to request verification of the arrest, as well as information about the charges that Heidari is facing.
I'll post more information as I receive it.
Cartoon by Hadi Heidari