CAIRO – Supporters of President Hosni Mubarak charged into Cairo's central square on horses and camels brandishing whips while others rained firebombs from rooftops in what appeared to be an orchestrated assault against protesters trying to topple Egypt's leader of 30 years. Three people died and 600 were injured.

The protesters accused Mubarak's regime of unleashing a force of paid thugs and plainclothes police to crush their unprecedented 9-day-old movement, a day after the 82-year-old president refused to step down. They showed off police ID badges they said were wrested from their attackers. Some government workers said their employers ordered them into the streets.

Mustafa el-Fiqqi, a top official from the ruling National Democratic Party, told The Associated Press that businessmen connected to the ruling party were responsible for what happened.

The notion that the state may have coordinated violence against protesters, who had kept a peaceful vigil in Tahrir Square for five days, prompted a sharp rebuke from the Obama administration.

"If any of the violence is instigated by the government, it should stop immediately," said White House Press Secretary Robert Gibbs.

The clashes marked a dangerous new phase in Egypt's upheaval: the first significant violence between government supporters and opponents. The crisis took a sharp turn for the worse almost immediately after Mubarak rejected the calls for him to give up power or leave the country, stubbornly proclaiming he would die on Egyptian soil.

His words were a blow to the protesters. They also suggest that authorities want to turn back the clock to the tight state control enforced before the protests began.

Mubarak's supporters turned up on the streets Wednesday in significant numbers for the first time. Some were hostile to journalists and foreigners. Two Associated Press correspondents and several other journalists were roughed up in Cairo. State TV had reported that foreigners were caught distributing anti-Mubarak leaflets, apparently trying to depict the movement as foreign-fueled.