Egypt's new president asserts authority Updated 10:20 PM ET CAIRO Egypt's new president moved to assert his authority and regain control of the streets Saturday even as his Islamist opponents declared his powers illegitimate and issued blood oaths to reinstate Mohammed Morsi, whose ouster by the military has led to dueling protests and deadly street battles between rival sides. But underscoring the sharp divisions facing the untested leader, Adly Mansour, his office said pro-reform leader Mohamed ElBaradei had been named as interim prime minister but later backtracked on the decision saying consultations were continuing. A politician close to ElBaradei said the reversal was due to objections by an ultraconservative Islamist party with which the new administration wants to cooperate. Mansour's administration, meanwhile, has begun trying to dismantle Morsi's legacy. He replaced Morsi's intelligence chief and the presidential palace's chief of staff. Prosecutors, meanwhile, ordered four detained stalwarts of Morsi's Muslim Brotherhood held for 15 days pending an investigation into the shooting deaths of eight protesters last week. No major violence was reported between supporters and opponents of Morsi as the two sides sought to regroup after a night of fierce clashes that turned downtown Cairo into a battlefield. Clashes were also fierce in the port city of Alexandria, where thousands from both sides fought each other with automatic rifles, firebombs and clubs. Friday's violence left 36 dead, taking to at least 75 the number of people killed since the unrest began on June 30, when millions of protesters took to the streets on the anniversary of Morsi's inauguration as Egypt's first democratically elected president. Morsi, a U.S.-trained engineer who was widely accused by critics of monopolizing power for himself and the Brotherhood as well as his failure to implement democratic and economic reforms, remained under detention in an undisclosed location. Tensions were still high as tens of thousands of Morsi supporters rallied for a third day near a mosque in a Cairo neighborhood that has traditionally been a stronghold of Islamists, chanting angry slogans against what they called a coup by Gen. Abdel-Fattah el-Sissi. The general has denied the military staged a coup, saying he was acting on the wishes of millions of Egyptians protesting the ex-Islamist leader. "El-Sissi is a traitor," declared an English language banner bearing an image of the army's chief and hoisted by Morsi's supporters. Setting up another showdown, the youth opposition group behind the series of mass protests that led to Morsi's ouster called on Egyptians to take to the streets on Sunday to show support for the new order. Mansour, 67, the former chief justice of the country's Supreme Constitutional Court who was installed by the military as an interim leader, is little-known in international circles and the choice of ElBaradei would have given his administration a prominent global face to make its case to Washington and other Western allies trying to reassess policies. But news of ElBaradei's appointment, which was reported by the state news agency MENA and others, proved divisive. The 71-year-old Nobel laureate was an inspiring figure to the youth groups behind the 2011 revolution that toppled autocrat Hosni Mubarak as well as the uprising against Morsi. His appointment as prime minister would cement Mansour's support among the young anti-Morsi protesters. But a senior opposition official close to ElBaradei, Munir Fakhry Abdelnur, told The Associated Press that the last minute reversal was because the ultraconservative Salafi el-Nour party was opposed. Mansour's spokesman Ahmed el-Musalamani denied that the appointment of the former U.N. nuclear negotiator was ever certain. However, reporters gathered at the presidential palace ahead of his news conference were told earlier that the president would arrive shortly to announce it.