There are reports that at least 42 people have been killed in an attack outside the headquarters of the Republican Guard in Cairo.
The military said supporters of former president Mohamed Morsi tried to storm the Egyptian army premises, where he is being held, but the Muslim Brotherhood claim their supporters were fired upon and "massacred" while praying outside the building.
The military said two of its officers were killed in the clashes when a "terrorist group" tried to storm the army base at dawn. The claims from both sides have not been independently verified.
Murad Ali, of the Brotherhood's Freedom and Justice Party, said shooting broke out while the Islamists had been staging a sit-in outside the barracks.
Some witnesses, including Brotherhood supporters at the scene, said the armed forces fired only warning shots and tear gas, and that "thugs" in civilian clothes approached from the side before carrying out the deadly shooting.
"The Republican Guard fired tear gas, but the thugs came from the side. We were the target," said Mahmud al Shilli, a Muslim Brotherhood protester.
"I saw with my own eyes the people who they shot at," another protester said, adding that several people had been injured.
Speaking to Sky's Sam Kiley, one witness said Morsi supporters were attacked from the side and that the army fired the first shots.
"They fired tear gas and live bullets, we saw several people drop dead in the street. The attack came from the other side.
"(the protesters) were not doing any harm. They were just making some prayers and singing songs; many of them are from outside Cairo.