Magdy Qorqor, a prominent Islamist leader and spokesman of the Muslim Brotherhood-led National Alliance to Support Legitimacy (NASL), was arrested on Monday in east Cairo, reported Reuter's Aswat Masriya.
Lawyer of the pro-Morsi Istiqlal Party Ashraf Omran told Aswat Masriya that security forces raided Qorqor's house in Nasr City and arrested him.
Omran added that both the charges against Qorqor and his detention venue are still unknown.
The arrest of the long-time Islamist New Labour Party member came a day after three Islamist party leaders were arrested.
On Tuesday, the NASL said security forces had arrested Nasr Abdel-Salam, acting head of the Building and Development Party, Magdy Hussein, head of the Istiqlal Party and Hossam Khalaf of the Wasat Party.
The NASL have called for mass protests on Thursday – a "day of rage" – to mark a year since Morsi's ouster and to break what they describe as a military coup that removed the Islamist president from power.