Eyewitnesses said 500 supporters of ousted president Mohammad Morsi are being besieged inside Al-Mostafa Mosque near the Republican Guards headquarters in eastern Cairo.
“Some 500 people, mostly women and children, are inside the mosque,” the eyewitnesses told Anadolu Agency.
“They are appealing for help because anyone who leaves the mosque is either shot or arrested,” they claimed.
The army has not commented on such reports.
The Egyptian Ambulance Authority (EAO) said 42 people were killed and 322 wounded outside the Republican Guards HQ in the early hours of Monday.
The Muslim Brotherhood said at least 53 people, including five children, were killed and hundreds wounded when army and police forces opened fire at peaceful protesters outside the HQ.
But the army insists that one officer was killed and several soldiers were wounded in an attempt by “a terrorist group” to storm the HQ.
Loyalists of Morsi, Egypt's first democratically elected president, have been camping outside the building where they think he was being held since his ouster under an army roadmap that suspended the constitution and named the head of the Constitutional Court as interim president.