CAIRO—At least 120 people were killed and 748 injured in early morning fighting between police and supporters of ousted President Mohammed Morsi, the Ministry of Health said, as Egypt's political divisions edged toward prolonged conflict. Saturday morning's violence was the deadliest single episode in the more than two years since Egypt's first revolution. The killings mark a dangerous escalation in a conflict that has already badly damaged Egypt's emerging democracy and ruptured a society that had once prided itself on its cohesiveness. The violence seemed to further polarize both sides of Egypt's ideological divide, and many appeared to be digging in for a prolonged showdown between supporters of Egypt's ousted president and security forces. "We are protesting and we will not give up," said Mourad Mohammed Ali, a former spokesman for Mr. Morsi's office and a leader in the Brotherhood. "We will continue fighting to get our freedom." The violence came hours after dueling protests brought Egyptian streets to a standstill as millions of Mr. Morsi's opponents staged a show of force following a call by army chief Gen. Abdel Fattah Al Sisi, who had asked Egyptians to protest Friday to give him a "mandate to confront terrorism"—a request widely thought to refer to Mr. Morsi's mostly Islamist backers.