An atmosphere of uneasy calm prevailed in Egypt on Saturday morning, following a night of nationwide clashes between supporters of ousted former President Mohammed Morsi and their military-backed opponents, which killed at least 30 people and injured more than 1,000, news agencies reported.

The Mediterranean city of Alexandria saw the deadliest violence, as 12 were killed and dozens were injured, while Cairo witnessed eight killings, according to BBC News, which cited Egypt’s health ministry.

Cairo’s Tahrir Square was largely empty on Saturday morning, after a massive rally on Friday by Morsi’s Islamist supporters, who demanded his reinstatement. The rally led to clashes between Islamists and their opponents, as fighters armed with stones, knives and petrol bombs continued to battle until late in the night, when the army was deployed to end the violence, Reuters reported.

The army-backed authorities, who ousted Morsi and suspended the country’s constitution, would appoint an interim prime minister on Saturday to be in charge until after parliamentary and presidential elections can be held, Reuters said, citing state-owned newspapers. Mohammed ElBaradei -- a Nobel Peace Prize laureate and former United Nations diplomat, as well as a prominent opposition leader -- is considered a top candidate to lead an administration that would shoulder the responsibility of restoring Egypt’s peace and security, and of reviving its ailing economy, according to Reuters.

The U.S. State Department condemned Friday’s violence and urged all parties involved to stop further aggression. “The voices of all who are protesting peacefully must be heard -- including those who welcomed the events of earlier this week and those who supported President Morsi,” State Department representative Jen Psaki said in a statement. “The Egyptian people must come together to resolve their differences peacefully, without recourse to violence or the use of force.”

Adly Mansour, head of Egypt’s Supreme Constitutional Court, was sworn in as the country’s interim president on Thursday, and he pledged to hold elections soon.

On Friday, Mansour dissolved the upper house of the parliament -- or Shura Council -- the only legislative body that was functional after the lower house was dismissed last year. He also appointed a new intelligence chief, Mohamed Ahmed Farid, after assuming power, BBC News reported.

Morsi is being detained, while one of the Muslim Brotherhood’s top leaders, Khairat el-Shater, was reportedly arrested in Cairo on Friday on suspicion of inciting violence.

Meanwhile, violence in the Sinai Peninsula, which shares a long border with Israel, killed five policemen, but the reason behind the killings remains unclear.

An Islamist group in the peninsula -- Ansar al-Shariah -- has said that the ouster of Morsi was a declaration of war on Islam, and it has threatened violence to impose Islamic law, Reuters reported.