Supporters of President Hosni Mubarak have opened fire on protesters in Cairo, killing up to four people and wounding 13 others.
It is the biggest spike in violence since protesters angered by oppression and hardship launched an unprecedented challenge to Mubarak ten days ago. Many accuse the government of backing the pro-Mubarak supporters.
Al Arabiya television quoted a doctor at the scene as saying four people had been killed and 13 wounded in the overnight violence in Tahrir Square which was shown live on television.
After Mubarak said in a nationally televised address on Tuesday that he would step down in September, the army told protesters to go home.
But with many saying they would not call off their protests until the 82-year-old president quit, Mubarak backers, throwing petrol bombs, wielding sticks and charging on camels and horses, attacked protesters in Tahrir Square.
Anti-Mubarak demonstrators say the attackers were police in plain clothes. The Interior Ministry has denied the accusation, and the government rejected international calls to end violence and begin the transfer of power.
This apparent rebuff along with the surge in violence, after days of relatively calm demonstrations, has complicated US calculations for an orderly transition of power in Egypt.
In pointed comments, a senior US official said it was clear that "somebody loyal to Mubarak has unleashed these guys to try to intimidate the protesters".
An estimated 150 people have been killed so far in protests across the country. United Nations human rights chief Navi Pillay said up to 300 people may have died.
