At the one of the fighting's front lines, next to the famed Egyptian Museum at the edge of Tahrir Square, pro-government rioters blanketed the rooftops of nearby buildings and dumped bricks and firebombs onto the crowd below - in the process setting a tree ablaze inside the museum grounds.
On the street, the two sides crouched behind abandoned trucks and pummeled each other with hurled chunks of concrete and bottles. Some among the more than 3,000 government supporters waved machetes as their anti-Mubarak rivals filled the air with a ringing battlefield din by banging metal fences with sticks.
The Health Ministry announced one dead - a soldier who fell off a nearby highway overpass - and more than 400 people injured. Bloodied young men staggered or were carried into makeshift clinics set up in mosques and alleyways by the anti-government side.
Protesters pleaded for protection from soldiers stationed at the square, who refused. Soldiers did nothing to stop the violence beyond firing an occasional shot in the air and no uniformed police were in sight. Some protesters wept and prayed in the square where around 10,000 had massed Wednesday morning and where only a day before they had held a joyous, peaceful rally of a quarter-million, the largest yet in more than a week of demonstrations demanding Mubarak leave power.
Protesters contended there were plainclothed police among their attackers, showing police ID badges they said were wrested off them. Others, they said, were paid by the regime to assault them - a tactic that security forces have used in the past.
"After our revolution, they want to send people here to ruin it for us," said Ahmed Abdullah, a 47-year-old lawyer in the square. "Why do they want us to be at each other's throats, with the whole world watching us."
Another man shrieked through a loudspeaker, "Hosni has opened the door for these thugs to attack us."
The clashes marked a dangerous new phase in Egypt's 9-day-old upheaval: the first significant violence between supporters of the two camps. Clashes began, first in the port city of Alexandria, just hours after Mubarak - the country's authoritarian ruler for nearly 30 years - went on national television Tuesday night and rejected protesters' demands he step down immediately. He defiantly insisted he would serve out the remaining seven months of his term.
That speech marked an abrupt shift in the deteriorating crisis. A military spokesman appeared on state TV Wednesday and asked the protesters to disperse so life in Egypt could get back to normal. That was a major turn in the attitude of the army, which for the past few days allowed protests to swell.
Also, the regime for the first time Wednesday began to rally its supporters in significant numbers to demand an end to the unprecedented protest movement.
Some 20,000 pro-government demonstrators held an angry but mostly peaceful rally across the Nile River from Tahrir, saying Mubarak's concessions were enough and demanding protests end now that he has promised not to run for re-election in September, named a new government and appointed a vice president for the first time.
Their gathering was shot through with bitterness at the jeers hurled against the 82-year-old Mubarak over the past nine days.
"I feel humiliated," said Mohammed Hussein, a 31-year-old factory worker. "He is the symbol of our country. When he is insulted, I am insulted."
Having the rival sides on the streets is particularly worrying because there do not appear to be anywhere near enough police or military to control resurgent violence. The anti-Mubarak movement has vowed to intensify protests to force him out by Friday, and the scenes of violence may have aimed to intimidate people from joining.
International concern was also mounting. A day after President Barack Obama pressed Mubarak to loosen his grip on power immediately, White House spokesman Robert Gibbs said the U.S. "deplores and condemns the violence that is taking place in Egypt" and called for restraint.
British Prime Minister David Cameron said Egyptian authorities must accelerate their political reforms and said that "if it turns out that the regime in any way has been sponsoring or tolerating this violence, that would be completely and utterly unacceptable." U.N. Secretary-General Ban Ki-moon, meeting Cameron in London, also condemned the violence as "unacceptable."
German Foreign Minister Guido Westerwelle said the assault on the protesters "raises the urgent question whether the political leaders of Egypt understand the need for rapid democratic reform."
The violence began after nearly 10,000 anti-government protesters massed again in Tahrir on Wednesday morning, rejecting Mubarak's speech as too little too late and renewing their demands he leave immediately.
The rally was peaceful, but Mubarak supporters began to gather at the edges of the square, and protesters formed a human chain to keep them out. In the early afternoon, around 3,000 pro-government demonstrators broke through and surged among the protesters, according to an Associated Press reporter at the scene.
They tore down banners denouncing the president, fistfights broke out, and protesters grabbed Mubarak posters from the hands of the supporters and ripped them to pieces.
From there, it escalated into outright street battles as hundreds poured in to join each side. They tore up chunks of pavement and grabbed ammunition from a nearby construction site, hurling stones, metal rods, concrete and sticks at each other and chasing each other.
At one point, a small contingent of pro-Mubarak forces on horseback and camels rushed into the anti-Mubarak crowds, trampling several and swinging whips and sticks. Protesters dragged some from their mounts, throwing them to the ground and beating their faces bloody. The horses and camels appeared to be ones used by the many touts around Cairo who sell rides for tourists.
The main battle line next to the Egyptian Museum - the famed treasury of pharaonic antiquities and mummies - surged back and forth repeatedly for hours. Anti-Mubarak protesters held up sheets of corrugated metal ripped from the construction site as shields from the hail of stones.
Some tried to charge into the buildings where government supporters on the roofs were pelting them with stones, but they were stopped by plainclothes security forces at the entrances. Several firebombs from the roof landed in the museum grounds, setting a tree ablaze. Soldiers tried to put it out with a hose.
Protesters were seen running with their shirts or faces bloodied. Scores of wounded were carried to a makeshift clinic at a mosque near the square and on other side streets. Doctors in white coats rushed about with bags of cotton, mercurochrome and bandages. One man with blood coming out of his eye stumbled into a side-street clinic.
As night fell, some protesters went to get food, a sign they plan to dig in for a long siege.
The army troops who have been guarding the square for days had been keeping the two sides apart earlier in the day, but when the clashes erupted they did not intervene. Most took shelter behind or inside the armored vehicles and tanks stationed at the entrances to the square.
"Why don't you protect us?" some shouted at soldiers, who replied they did not have orders to do so and told people to go home.
"The army is neglectful. They let them in," said Emad Nafa, a 52-year-old among the protesters, who for days had showered the military with love for its neutral stance.
The new tensions began to emerge immediately following Mubarak's speech Tuesday night. Later in the night, clashes erupted between pro- and anti-government demonstrators in the Mediterranean city of Alexandria, while in Cairo groups of Mubarak supporters took to the streets, some carrying knifes and sticks.
Gatherings of Mubarak supporters were more hostile to journalists and foreigners. Two Associated Press correspondents and several other journalists were roughed up during various such gatherings. State TV reported Tuesday night that foreigners were caught distributing anti-Mubarak leaflets, apparently trying to depict the movement as foreign-fueled.
The violence could represent a dangerous new chapter after a series of dramatic and unpredictable twists in Egypt's upheaval.
After years of tight state control, protesters emboldened by unrest in Tunisia took to the streets on Jan. 25 and mounted a once-unimaginable series of demonstrations across this nation of 80 million. Initially, police cracked down hard with deadly assaults on the demonstrators. Then police withdrew completely from the streets for the day, opening a wave of looting, armed robberies and arson - largely separate from the protests themselves - that stunned Egyptians.
But since Sunday, the army moved in to take control and the situation became more peaceful. The military announced it would not stop protests. As a result, the demonstrations swelled dramatically, protesters gained momentum and enthusiasm and many believed Mubarak's immediate fall was at hand. The United States put intense pressure on Mubarak to bring his rule to an end while ensuring a stable handover.
Wednesday's events could mean the regime has had enough, and that it and the military aim to ensure the end of the unrest to let Mubarak shape the transition as he choses over the next months. Mubarak has offered negotiations with protest leaders over democratic reforms.
As if to show the public the crisis was ending, the government began to reinstate Internet service after days of an unprecedented cutoff. State TV announced the easing of a nighttime curfew, which now runs from 5 p.m. to 7 a.m. instead of 3 p.m. to 8 a.m.
Mubarak supporters were on the street in significant numbers for the first time on Wednesday. Across the Nile River from the chaos in Tahrir Square, around 20,000 pro-government demonstrators held a rally in front of Mustafa Mahmoud Mosque in the upper-class neighborhood of Mohandiseen, after notices on state TV calling for attendance.
They waved Egyptian flags, their faces painted with the black-white-and-red national colors, and carried a large printed banner with Mubarak's face as police officers surrounded the area and directed traffic. They cheered as a military helicopter swooped overhead.
Some appeared to be the sort of young toughs that the opposition accuses the regime of paying to be its fist in the streets.
But the large majority were middle-class families, some of whom said Mubarak's concessions were enough and that they feared continued instability and shortages of food and other supplies if protests continue.
"I want the people in Tahrir Square to understand that Mubarak gave his word that he will give them the country to them through elections, peacefully, now they have no reason for demonstrations," said Ali Mahmoud, 52, who identified himself as middle-class worker from Menoufia, a Nile Delta province north of Cairo.
The movement against Mubarak, meanwhile, was working to prevent any slipping in its ranks after the speech and resist any sentiment that the concession may have been enough.
One protest organizer said the regime was going all out to pressure people to stop protesting.
"Starting with the emotional speech of Mubarak, to the closure of banks, the shortage of food and commodities and deployment of thugs to intimidate people, these are all means to put pressure on the people," said Ahmed Abdel-Hamid, a representative of the Revolutionary Committee, one of several youth groups that organized the protests.
The movement is fueled by deep frustration with an autocratic regime blamed for ignoring the needs of the poor and allowing corruption and official abuse to run rampant. Tuesday's massive rally in Tahrir showed a large cross-section of Egyptian society.
In his 10-minute speech Tuesday night, Mubarak emphasized the theme that he has often used in justifying his rule during his nearly three decades in power - that he can keep stability. Now he was promising to do so as he heads out the door.
The president, who almost never admits to reversing himself under pressure, insisted that even if the protests demanding his ouster had not broken out, he would not have sought a sixth term in September.
Somber but firm - without an air of defeat - he said he would serve out the rest of his term working "to accomplish the necessary steps for the peaceful transfer of power." He said he will carry out amendments to rules on presidential elections.
AP correspondents Hamza Hendawi, Diaa Hadid, Lee Keath, Michael Weissenstein and Maggie Michael contributed to this report.