CAIRO: Mahienour El-Massry "was arrested as soon as she left the State Security Prosecutor´s headquarters in Cairo, where she had attended the investigations as a lawyer for several of those arrested during the demonstrations", Tarek al-Awadi said.
Hundreds of Egyptians took to the streets in Cairo and several other cities across the country on Friday to call for Sisi´s departure.
According to the Egyptian Centre for Economic and Social Rights, 365 people have since been arrested.
Massry, a political activist and human rights defender, has been tried and jailed twice for taking part in demonstrations.
In December 2013, following the army´s overthrow of Islamist president Mohamed Morsi following mass protests, she was arrested and imprisoned until September 2014 on charges of participating in a demonstration without a permit.
While in prison, Massry received the Ludovic Trarieux Award, an international prize given out annually to a lawyer for contributions to human rights.
In 2015, Massry was given another year-long sentence for taking part in a sit-in during Morsi´s rule.
Egypt effectively banned protests under a law passed following Morsi´s 2013 ouster.
Sisi was elected president the following year with 96.9 percent of the vote.