The family of ousted president Mohamed Morsi said Monday they plan to sue Egypt's army chief, General Abdel Fattah al-Sisi, for having "kidnapped" the elected Islamist head of state.
Hundreds of Morsi's supporters, meanwhile, marched to the public prosecutor's office and the defence ministry in Cairo, chanting anti-military slogans and carrying pictures of the toppled president.
On Sunday, the interim cabinet held its first meeting and urged all parties to keep protests peaceful after weeks of demonstrations across the country and a day of violence in Sinai that killed four security forces personnel.
Shaimaa Mohamed Morsi, the toppled president's daughter, told a press conference that the family were planning to take legal action in Egypt and abroad.
"We are taking local and international legal measures against Abdel Fattah al-Sisi, the leader of the bloody military coup, and his putschist group," she said.
She voiced dismay at "the silence of rights organisations and civil society over the crime of kidnapping the legitimate president," who was voted into office last year.
The family held Sisi and his "putschist group" responsible for Morsi's safety.
Morsi has been detained at an unknown location since his overthrow by the army on July 3 following mass protests against his presidency across the country.
Morsi's son Osama told reporters the family had not heard from him since his overthrow. "None of us has had any contact with our father since the afternoon of the coup on July 3," he said.
Several countries, including the United States and Germany, have called for his release, but the country's interim authorities have rejected the calls, saying he is being held in a "safe place".
Supporters of Morsi, who was ousted after a single turbulent year of rule following a presidential election, have pressed demonstrations, holding marches and protests across the country since his fall.
They pressed their demands for Morsi's reinstatement on Monday.
Members of the Shura council, Egypt's upper chamber, held a defiant meeting in Cairo's Rabaa al-Adawiya mosque on Monday.
The council was overwhelmingly dominated by Islamists under Morsi's presidency, and held all legislative powers. But Egypt's new authorities dissolved the council when they toppled Morsi.
Hundreds of the ousted president's backers also marched to the public prosecutor's office and the defence ministry on Monday.
Chanting "Sisi killer," and anti-police slogans, demonstrators hung pictures of the ousted president on the gates of the public prosecutor's office.
"I believe we will restore him by more pressure on the streets," Mohammed Awad, one of the protesters, told AFP.
Others at the march denounced the new interim authorities' legitimacy.
Supporters of Morsi also rallied in the Abbasiya area of Cairo, near the defence ministry.
On Sunday, the cabinet meeting focused on Egypt's battered economy and efforts to boost the security situation in the country, which has been increasingly precarious since Morsi's ouster.
Attacks in the Sinai peninsula killed four security services personnel and two civilians earlier the same day.
Assailants shot dead the three soldiers and one policeman in separate attacks in the town, and two civilian bystanders were killed later when the army traded fire with a group of gunmen, security sources told AFP.
In Rafah, a Sinai town that straddles the border with the Gaza Strip, unknown assailants launched an attack on a Central Security (riot police) camp with mortars and rocket-propelled grenades, wounding five policemen.
Analysts attribute the Sinai violence to Islamist extremists seeking to take advantage of the political insecurity in the country after Morsi's ouster.
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