The April 6 Youth Movement said in a statement on its official Facebook page that its co-ordinator Amr Ali was arrested at his home in Monufiya province, north of Cairo, and held at a state security building, the DPA reported.
State-run newspaper al-Ahram quoted a local detective chief as saying that Ali was being interrogated. The officer did not indicate what offences he was suspected of.
Many of Egypt's most prominent democracy activists have been jailed since then army chief Abdel-Fattah el-Sisi - who has since been elected president - deposed Islamist president Mohammed Mursi in 2013.
Ali's predecessor as head of the April 6 Youth Movement, Ahmed Maher, is currently serving a three-year jail term for taking part in a protest.