TEHRAN — Iranian security forces arrested a prominent human rights activist, Narges Mohammadi, on Tuesday, the reformist Baharnews website reported. Ms. Mohammadi is one of the best-known activists remaining in Iran.

For more than a decade, Ms. Mohammadi, 43, has worked with the Defenders of Human Rights Center. The group was founded by Shirin Ebadi, who received the Nobel Peace Prize in 2003. Political pressure forced Ms. Ebadi to leave Iran, and she now lives in Switzerland.

This week, Ms. Mohammadi, 43, who has been sentenced to prison terms on multiple occasions for her human rights work, was scheduled to appear in court in connection with a new case filed against her by the Iranian authorities. A request for a delay in that case was denied, leaving her lawyer no time to study the charges against her, Baharnews reported.

“We don’t know where she is as of this morning, and we don’t know where the security forces have taken her,” said a friend, who spoke on the condition of anonymity because of the delicacy of the case.