More than 30 people were killed and more than 1000 injured in Friday's violence following the ousting of Egypt's President Mohammed Morsi, it has emerged.
At least 12 died in Alexandria, and eight in two separate clashes in Cairo, the Health Ministry 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.
The BBC reports that some senior figures of his Muslim Brotherhood movement have also been held.
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.
The Tamarod [Rebel] movement - which organised recent anti-Morsi protests - accused the ousted president of pursuing an Islamist agenda against the wishes of most Egyptians, and of failing to tackle economic problems.
The US State Department issued a condemnation of Friday's violence and called for all leaders to put a stop to any further aggression.
UN Secretary General Ban Ki-moon has also expressed alarm at the violence, saying that it was for the people of Egypt to determine the way forward - and all people, including women, needed to be part of that process.
He referred to "horrifying reports of sexual violence".
Most of those killed during fighting in the Mediterranean city of Alexandria, died from gunshot wounds, said Amr Nasr, head of emergency services in the city.
He told the official Mena news agency that 200 people were injured during clashes in Egypt's second-largest city.
The scenario many had feared materialised on Friday evening. There were running battles in Cairo between those who oppose Mohammed Morsi and those who support him, as well as clashes in other parts of Egypt.
