At least 54 people were killed and over 400 wounded in a clash outside a military building in Cairo where Muslim Brotherhood affiliated protesters were demanding the return of Mohammed Morsi to power.

Monday’s incident has the highest number of fatalities since public protests lead the military to remove Morsi and establish an interim civilian administration.

The interim administration released a statement expressing regret for the deaths but said that protesters should not approach military or “other vital installations.” Police reported that the military troops came under “heavy gunfire” in the middle of the night as protesters opened fire from rooftops and throwing Molotov cocktails.

Egypt’s top Muslim cleric warned of “civil war” and was going into hiding “as a protest” to both sides. He said he had no choice until “everyone shoulders his responsibility to stop the bloodshed instead of dragging the country into civil war.”

The Muslim Brotherhood’s political arm has called on all Egyptians to rise up against the army.