"Sixteen people were killed and a hundred others injured, many of them in serious condition," the Muslim Brotherhood's spokesperson Ahmed Aref said on Monday.
Police barricades prevented journalists from accessing the area.
Meanwhile, Egyptian troops and police early on Monday used guns and teargas to disperse Islamist supporters of ousted president Mohamed Morsi outside the Republican Guard headquarters in Cairo, the protesters said.
"I saw with my own eyes the people who they shot at" one protester said, adding that several people had been injured.
A fellow protester said the security forces fired bullets into the air and threw several teargas grenades.
"They wanted to disperse the protesters" who were determined to stage an indefinite sit-in, according to the protester.
On Friday at the same spot, four Islamists were shot dead by soldiers.
Supporters of Morsi's Muslim Brotherhood have mobilised en masse in different parts of Cairo in recent days, vowing to defend Morsi who was ousted by the military on Wednesday after mass protests calling for him to be removed from power.
Morsi was Egypt's first democratically elected leader but lasted just a year in power. – AFP