Violent clashes broke out between pro-and anti-Hosni Mubarak supporters and Army fired warning shots in the air, a day after a defiant Egyptian President said he would not seek another term in September but the protesters insisted he quit immediately.

A dramatic and potentially deadly situation unfolded in which petrol bombs or Molotov cocktails were hurled by anti-Mubarak supporters from the edge of Tahrir Square in central Cairo, the epicentre of the nine-day protests against the President's 30-year-old rule.

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According to eyewitnesses, the warning shots were fired in the air by the troops at the main rally against Mubarak and it is thought to be the first time they have resorted to this action since they were deployed on Friday.

For the first time, 82-year-old Mubarak's supporters took to the streets in central Cairo after they broke through the rally by protesters and urged the President not to quit under any circumstances.

In a televised address, Mubarak said he would not seek a sixth term in September, but also indicated he would not cede the presidency immediately, eliciting boos and chants of "Leave, leave" from protesters in central Cairo.

His remarks on Tuesday night did little to appease the crowds still gathered in Tahrir Square, a focal point for demonstrations in the Egyptian capital.

As midnight neared, they insisted they would not leave until Mubarak stepped down.

A military spokesman appeared on state TV and asked the protesters to disperse so life in the most populous Arab nation could get back to normal.

The United States condemned the violence and repeated its call for restraint.

"The United States deplores and condemns the violence that is taking place in Egypt, and we are deeply concerned about attacks on the media and peaceful demonstrators," White House spokesman Robert Gibbs said in a statement.

Egypt's army, hugely popular with the public, has so far refrained from interfering with the huge protests and the marches by the people and it was not immediately known whether its new warnings were a prelude to any clampdown.

The army's warning came as Mubarak, buckling under pressure, promised not to stand for presidential elections scheduled in September, but said he had no intention to flee Egypt.