Thousands of Egyptians gathered after Friday prayers and fought battles with lines of riot police at all the major junctions of the capital.
Riot squads gave way to armoured personnel carriers as the crowds ignored a curfew imposed at dusk.
Police managed to keep the crowds from Tahrir Square – Liberation Square – in the city centre by cutting them off from each other at every major junction.
But by nightfall the situation threatened to spiral out of control as protesters defied a nationwide curfew.
The headquarters in central Cairo of the governing party was set on fire and a crowd surrounded the central radio and television building. 870 were injured, several with bullet wounds.
Hillary Clinton, the US Secretary of State, led calls for the authorities to show restraint, saying that the protests "underscore that there are deep grievances within Egyptian society, and the Egyptian government needs to understand that violence will not make these grievances go away."
"We are deeply concerned about the use of violence by Egyptian police and security forces against protesters," Mrs Clinton told reporters.
William Hague, the Foreign Secretary, urged the Egyptian government to heed the "legitimate demands" of protesters.
He said it was "impossible to predict what the outcome will be of a long period of disorder, of violence or attempted revolution".
Mr Hague added: "The right response of countries concerned with the situation, not just Egypt, is to respond to the legitimate demands of economic development and a more open and flexible political system."
Dozens of British tourists caught in Cairo were warned not to leave their hotels by the Foreign Office during the continuing unrest.
Overnight on Thursday the Egyptian authorities closed down first Facebook and Twitter, then the internet and finally, on Friday morning, the mobile phone network.
In doing so they managed to split the opposition movement but encouraged greater violence as initially peaceful chanting gave way to frustrated mobs charging the lines with stones and petrol bombs.
The protests which began last Tuesday had become quieter by Thursday but a Facebook page with 91,000 followers had begun calling for a gathering outside major mosques across the country on Friday, the Muslim day of rest.
Mohamed ElBaradei, the former head of the International Atomic Energy Agency whose calls for reforms has made him a figurehead for opposition to Mr Mubarak, came under fire from tear gas after praying in public outside a mosque near his home in Giza, the suburb near the Pyramids.
He was hustled away inside the mosque and was later reported to be under house arrest.
As protesters squared up against the lines of riot police, they shouted slogans calling for Mr Mubarak to go – along with his unpopular son, Gamal, whom many think Mr Mubarak is grooming to succeed him.
"People want the regime to fall," was one chant, the same that had been levelled at President Zine al-Abedine Ben Ali of Tunisia two weeks before to the day.
Anti-government protestors in Egypt rallying against the rule of president Hosni Mubarak took to the streets for a fourth straight day, demanding the long-time leader step down and complaining of poverty, corruption, and oppression. The demonstrations were supposedly inspired by a popular uprising that saw Tunisian president  ousted from office. A national curfew is in effect, but is being ignored by many.
Protestors in Cairo, Tuesday – the initial day of protests. Tens of thousands of people spilled out into streets after Friday prayers today, chanting slogans like "Down, down with Mubarak" and "The people want the regime to fall". At least eighteen people died in the violence between riot police and protestors, with hundreds more wounded. Up to a thousand people have been arrested.

Police responded to protestors by deploying rubber bullets, tear gas, and water cannons. In addition, Mubarak ordered the army out onto the streets to help quell the conflict. He was earlier supposed to make a statement, but hasn't yet done so.

In Cairo, demonstrators targeted the heavily-guarded state broadcasting building, as well as the building of the ruling , which was set on fire, according to a BBC reporter. In , armed men took several policemen hostage after seizing a road leading into a nearby town. 

Former International Atomic Energy Agency head Mohamed ElBaradei, a prominent opposition figure, was taken into custody by authorities and now under house arrest, '''' reports.

=== Internet shutdown ===

Meanwhile, according to media reports and statistics, Egypt has shut down practically all Internet traffic both out of and into the nation, as well as disrupting cellphone usage. A graph by Arbor Networks available on the '''' website shows an almost vertical drop in overall Egypt internet traffic, starting shortly before four o'clock in the afternoon with over 2.5 million megabits per second down to almost zero in the space of an hour.

"Almost nobody in Egypt has Internet connectivity, and there are no workarounds. I’ve never seen it happen at this scale," said the chief technology officer for Renesys, a company that tracks Internet traffic. "In a fundamental sense, it’s as if you rewrote the map and they are no longer a country. I never thought it would happen to a country the size and scale of Egypt."

Andrew Noyes, spokesman for Facebook, noted that the social website had seen almost all of its Egypt traffic disappear and there is only a "minimal" amount left. In a statement, Noyes said: "Although the turmoil in Egypt is a matter for the Egyptian people and their government to resolve, limiting Internet access for millions of people is a matter of concern for the global community."

 announced on its website that "all mobile operators in Egypt have been instructed to suspend services in selected areas. Under Egyptian legislation the authorities have the right to issue such an order and we are obliged to comply with it."