CAIRO — Overnight clashes in Cairo between supporters and opponents of Egypt's Islamist leader killed at least three people, the Interior Ministry said Thursday, as the nation further descended into turmoil over the constitution drafted by President Mohammed Morsi's allies.

The street battles outside the presidential palace in the city's Heliopolis district were the worst violence since Egypt's latest political turmoil erupted Nov. 22, when Morsi assumed near absolute powers.

The intensity of the fighting marked a milestone in a schism that is pitting Morsi's Muslim Brotherhood and ultraconservative Islamists in one camp, against liberals, leftists and Christians in the other.

Morsi, for his part, pressed forward with plans for a Dec. 15 constitutional referendum to pass the new constitution.

It was the first time supporters of rival camps have fought each other since last year's uprising that toppled authoritarian ruler Hosni Mubarak.

A statement from the ministry Thursday said 276 people were injured in the clashes, including 35 policemen, as mobs battled each other with firebombs, rocks and sticks outside the presidential complex.

The fighting erupted Wednesday afternoon when thousands of Morsi's Islamist supporters massed near the presidential palace where some 300 of his opponents were staging a sit-in. The Islamists, members of the Muslim Brotherhood, chased the protesters away from their base outside the palace's main gate and tore down their tents.

After a brief lull, hundreds of Morsi opponents arrived and began throwing firebombs at the president's backers, who responded with rocks.

Compounding Morsi's woes, four more presidential aides resigned Wednesday, joining two other members of his 17-member advisory panel who have abandoned him since the crisis began.

The violence spread to other parts of the country Wednesday. Anti-Morsi protesters stormed and set ablaze the Brotherhood offices in Suez and Ismailia, east of Cairo, and there were clashes in the industrial city of Mahallah and the province of Menoufiyah in the Nile Delta north of the capital.

Mohamed ElBaradei, a leading opposition advocate, said Morsi's rule was "no different" from Mubarak's.

"In fact, it is perhaps even worse," the Nobel Peace Prize laureate said at a news conference after he accused the president's supporters of a "vicious and deliberate" attack on peaceful demonstrators outside the palace.