Adly Mansour , Egypt 's new interim president, swore the oath of office in front of Egypt 's Supreme Constitutional Court on Thursday morning, ending a whirlwind insurrection that brought millions of Egyptians onto the streets and overturned an Islamist president that only days ago had seemed immutable.

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.

Highlighting the haste of
Egypt 's second revolution in more than two years, Mr. Mansour took two oaths on Thursday morning: The first made him chief justice of the Supreme Constitutional Court while 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. 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.