CAIRO (AP) — Egyptian President Mohammed Morsi is refusing to step down despite massive protests and his supporters are vowing to prevent his ouster.

Hundreds of thousands of anti-Morsi protesters thronged the streets of Cairo and cities around the country Sunday and marched on the presidential palace in an attempt to force out the Islamist president. The demonstrations are the largest 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 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 say at least three protesters were killed.

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