Egyptian state prosecutors on Friday remanded Masoum Marzouk, a prominent opposition figure and former diplomat, along with six others, in custody for 15 days, pending investigation into charges against them. Those charges include "inciting unrest" and "membership in a terrorist organization," Tarek al-Awadi, a lawyer for the defendants, told local media outlets.

On Thursday evening, security forces detained Marzouk, economist Raed Salama, academic Yehia al-Qazzaz, and four other critics of the regime of President Abdel-Fattah el-Sissi.

Earlier this month, Marzouk had called for a referendum and proposed a political road map if Egyptians voted against the government. The transition would have included the suspension of the constitution, the termination of the current presidential term and the dissolution of parliament, which is packed with el-Sissi supporters. Marzouk said if the government rejected his proposal he would hold a "popular conference" in Tahrir Square – epicenter of the 2011 uprising – on Aug. 31.

A vocal critic of the current regime, Marzouk held several diplomatic posts under former President Hosni Mubarak, who was forced to step down, after 30 years in power, following a 2011 popular uprising.

El-Sissi came to power in 2013 on the back of a bloody military coup against Mohammed Morsi, who became Egypt's first-ever freely-elected leader in a presidential poll held one year earlier. Morsi, along with hundreds of supporters and members of his banned Muslim Brotherhood group, has remained behind bars since the 2013 coup.