Backers of President Hosni Mubarak, throw petrol bombs, wield sticks and charge on camels and horses, attacked protesters in Cairo on Wednesday after the army told reformists demanding the president quit to go home.
Anti-Mubarak demonstrators hurled stones back and said the attackers were police in plainclothes. The Interior Ministry denied the accusation, and the Egyptian government rejected international calls for Mubarak to end his 30-year rule now.
In pointed comments, a senior U.S. official said it was clear that "somebody loyal to Mubarak has unleashed these guys to try to intimidate the protesters."
Troops and tanks stood by as the violence raged.
The emergence of Mubarak loyalists, whether ordinary citizens or police, injected a new dynamic into the momentous uprising in this most populous Arab nation of 80 million people.
As night fell, Egyptian Vice President Omar Suleiman urged the 2,000 demonstrators bedding down in Cairo's central Tahrir (Liberation) Square to leave and observe a curfew to restore calm. Suleiman said the start of dialogue with the reformists and opposition depended on an end to street protests.
But protesters barricaded the square against pro-Mubarak supporters trying to penetrate the makeshift cordon, and also conducted searches. There was sporadic gunfire, with blazes caused by firebombs, and the atmosphere was tense.
"This place will turn into a slaughterhouse very soon if the army does not intervene," Ahmed Maher, who saw aggressive pro-Mubarak supporters with swords and knives, told Reuters.