Pro-Morsi factions overran about 200 protesters camped outside the presidential palace in north Cairo. The clashes came after the Muslim Brotherhood's Freedom and Justice Party called thousands of its members into the streets in a counter-demonstration to drive opposition movements from the presidential palace.

Shoving and punching spilled down a boulevard as hurled stones, swinging sticks and firebombs filled the dusk in one of the capital's wealthiest neighborhoods. Pro-Morsi contingents, including Brotherhood followers and ultraconservative Salafis, chased opposition activists, shouting: "God is great! The people support the president's decision!"

More than 200 people were injured across a cityscape that had the charged air of a fluorescent-lit battlefield with competing banners, bandaged men and dinner trays used as shields to block barrages of rocks. Egyptian news reports said clashes spread to other cities, including attacks on several Muslim Brotherhood offices. There were unconfirmed reports of at least three deaths.

Police were slow to react in Cairo but eventually arrived and attempted to separate the two sides, whose skirmishes raised fears that animosity between Islamists and the mainly secular opposition were a dangerous foreshadowing. Both camps threatened marches, and there appeared little compromise in a battle over the nation's future that symbolizes the larger struggle over political Islam rising from the "Arab Spring."

"Vicious attack vs. peaceful protesters in front of presidential palace," tweeted Mohamed ElBaradei, a Nobel Peace Prize laureate and opposition leader. "Regime leading Egypt into violence and bloodshed."

The clashes exposed new fissures in Morsi's government from officials disturbed by the power and meddling of the Brotherhood. Three presidential advisors resigned, including Seif Abdel-Fattah, who told the Egyptian news media: "Egypt is bigger than a narrow-minded elite.... We can no longer stay silent because they [the Brotherhood] have harmed the nation and the revolution."

The defections further marred the credibility of Morsi's administration at a time when Egypt's political polarization and continuing unrest have alarmed the international community. The White House, which praised Morsi for his role in negotiating a cease-fire between the militant group Hamas and Israel in the Gaza Strip, is pressing Cairo to calm the crisis.