Twelve people have died in Alexandria and three in Cairo in clashes between supporters and opponents of ousted President Mohammed Morsi
The violence came after three pro-Morsi demonstrators were killed by security forces in another part of the capital.
Troops later restored calm in Cairo, but nationwide violence left some 26 dead and 318 injured, officials said.
The army removed Mr Morsi from power on Wednesday after millions of people protested over his leadership.
Mr Morsi, Egypt's first freely elected leader, is in detention, as are some senior figures of his Muslim Brotherhood movement.
Early on Saturday, state media reported the Brotherhood's deputy leader Khairat el-Shater had been arrested at his Cairo home on suspicion of incitement to violence.