In a sharp escalation of tensions Monday, Egyptian soldiers opened fire on hundreds of supporters of Mohamed Morsi, the ousted president.    

The Al Nour party, seen as a potential kingmaker in Egyptian politics, announced its decision after what it called a “massacre” of protesters on Monday in Cairo.    

Al Nour, a party of ultraconservative Islamists, has emerged as an unexpected political kingmaker, shaping the interim government after the ouster of President Mohamed Morsi.    

American diplomats contacted Muslim Brotherhood leaders to try to persuade them to re-enter the political process, an Islamist briefed on one of the conversations said.