Associated Press:

Hundreds of thousands thronged the streets of Cairo and cities around the country Sunday and marched on the presidential palace, filling a broad avenue for blocks, in an attempt to force out the Islamist president with the most massive protests Egypt has seen in 2½ years of turmoil.

In a sign of the explosive volatility of the country's divisions, young protesters mainly from the surrounding neighborhood pelted the main headquarters of President Mohammed Morsi's Muslim Brotherhood with stones and firebombs, and at one point a fire erupted at the gates of the walled villa. During clashes, Brotherhood supporters barricaded inside opened fire on the attackers, and activists said at least three protesters were killed.

At least five more anti-Morsi protesters were killed Sunday in clashes and shootings in southern Egypt.

Fears were widespread that the collisions between the two sides could grow more violent in coming days. Morsi made clear through a spokesman that he would not step down and his Islamist supporters vowed not to allow protesters to remove one of their own, brought to office in a legitimate vote. During the day Sunday, thousands of Islamists massed not far from the presidential palace in support of Morsi, some of them prepared for a fight with makeshift armor and sticks.

Protesters have vowed to stay on the streets until he steps down, and organizers called for widespread labor strikes starting Monday.