The Egyptian army has fired warning shots for the first time during bloody clashes in Cairo.
Thousands of people, from opposing groups, have been attacking each other in the streets of the capital around Tahrir Square.
At one point a small contingent of pro-Mubarak forces on horseback and camels rushed into the anti-Mubarak crowds, swinging whips and sticks.
Protesters retaliated, dragging some from their mounts, throwing them to the ground and beating them.
Protesters were seen running with their shirts or faces bloodied.
Tear gas was believed to have been deployed, though it was not clear who had fired it.
The troops who have been guarding the square had been keeping the two sides apart earlier in the day, but when the clashes began they did not intervene.
Most took shelter behind or inside the armoured vehicles and tanks stationed at the entrances to Tahrir.
However, as the violence escalated, soldiers were ordered to fire warning shots into the air.
The military had called for an end to more than a week of demonstrations demanding Mubarak steps down immediately after nearly 30 years in power.
"Your message has arrived, your demands became known," military spokesman Ismail Etman said on state television in an address directed to young protesters. "You are capable of bringing normal life to Egypt."
Mr Mubarak's supporters were out in the streets for the first time today in large numbers, with thousands demanding an end to the anti-government movement a day after the president went on national television and rejected demands for him to step down.
Those calling for Mr Mubarak to go have been out in Cairo and many other cities for more than a week, and they drew by far their largest crowd yesterday when at least 250,000 packed Tahrir Square and the area around it.
Today, Mr Mubarak's supporters were seen breaking through a human chain of anti-government protesters trying to defend thousands gathered in Tahrir.
They tore down banners denouncing the president and fist-fights broke out as they advanced across the massive square in the heart of the capital.
The anti-government protesters grabbed Mr Mubarak posters from the hands of the supporters and ripped them.
The confrontations began just hours after a military spokesman went on national television and asked the protesters to disperse so life in Egypt could go back to normal.
Almost immediately after Mr Mubarak announced late last night that he would serve out the remaining seven months of his term - but would not seek re-election in September - and would not leave the country, groups of his supporters rarely seen before in the week of anti-government protests took to the streets, some carrying knives and sticks.
In Alexandria earlier, clashes erupted between the pro and anti-Mubarak protesters.