Cairo: The family of ousted president Mohammed Mursi has vowed to press charges against the powerful military commander who forced him from power.

Osama Mursi, the son of the former president, spoke at a news conference in downtown Cairo on Monday, as hundreds of demonstrators massed in front of the attorney general's office down the street, chanting against the military officers who removed Dr Mursi, Egypt's first democratically elected president, in a July 3 coup.

Osama Mohamed Mursi, son of Egypt's ousted president Mohamed Mursi, speaks during a press conference in Cairo. Credit:AFP

The younger Mursi called his father's detention by the country's armed forces a "kidnapping" and a "crime." He said the secretive conditions of the detention violated the standards of international law and underscored the "militarisation" of Egypt since the coup, despite the appointment of a civilian president and prime minister.

"I challenge the attorney general to [show he] knows where president Mursi is being detained," Osama Mursi said.