Adly Mansour, Egypt’s new interim president, swore the oath of office in front of Egypt’s Supreme Constitutional Court on Thursday morning, formalizing a whirlwind insurrection that brought millions of Egyptians onto the streets and overturned an Islamist president that only days ago 
had seemed immutable, the Wall Street Journal reported.
Mr. Mansour swore his oath hours after Egyptian security forces moved briskly to arrest and detain hundreds of Muslim Brotherhood leaders—the powerful Islamist organization that has dominated Egyptian elections over the past two years.
Mohammed Morsi, Egypt’s ousted president, was reportedly in police custody on Thursday morning after he refused to step down from his post on the orders of Gen. Abdel Fattah Al Sisi on Wednesday night.
Mansour took two oaths on Thursday morning: The first made him chief justice of the Supreme Constitutional Court and the second elevated him to the presidency.
Mr Mansour has been the head of Egypt’s Supreme Constitutional Court since Monday, but the court’s own swearing-in ceremony was delayed for several days due to massive street protests against Mr. Morsi’s rule.
The leader of Egypt’s military ousted Mr. Morsi from office and replaced him with Mr. Mansour late Wednesday evening. The presidential palace immediately branded the move a “complete military coup.”
The announcements capped days of political crisis that brought millions of Egyptians out to the country’s streets, spurring bellicose rhetoric from Mr. Morsi’s backers and Egypt’s military, and sparking deadly violence. Ten people were killed and 481 injured in clashes around the country on Wednesday, the health ministry said.
Two years after the biggest tremor of the Arab Spring ousted Hosni Mubarak, and a year after his elected replacement took office, Egyptian streets again thronged with protesters calling for the removal of a despot, the stark divides between their celebration and anger suggesting a new period of political uncertainty lies ahead.
Egypt’s acting attorney general on Thursday issued arrest warrants for Muslim Brotherhood supreme leader Mohamed Badie and his deputy Khairat al Shater on charges of inciting the killing of protesters, according to MENA, the state news agency.
The two are wanted on charges of inciting the killing of eight protesters in front of the Muslim Brotherhood’s headquarters in Cairo. The prosecutor’s decision came after eyewitnesses in the neighborhood said they were able to testify against the Brotherhood leaders, according to MENA.
The military overthrow poses a new challenge for Washington. The US has a deep relationship with Egypt’s military, which is budgeted to receive some $1.3 billion in US aid this year. But congressional legislation demands the US suspend assistance to allied militaries that are certified as having overthrown democratically elected governments.
Two powerful US senators on a visit with troops in Afghanistan on Thursday affirmed that continued assistance from Washington would hinge on a swift restoration of civilian rule.
“We view with great concern the events that are taking place in Egypt,” said Sen. John McCain (R., Ariz.). “I believe that the defense minister and the military have to show us and the world that they are making a rapid transition back to democracy. And that will be, I think, the indicator to the level of support that we would provide to Egypt.”
Sen. Lindsey Graham (R., S.C.), who serves on the Senate Appropriations Committee, said the Egyptian military had to abide by the rule of law as a condition of aid.
“If the democratically elected government is overthrown by the military, you would suspend all assistance,” he said. “It looks like a coup, it sounds like a coup, but I’m in the camp with Sen. McCain: Let’s look and see how this unfolds.”