At least one protester was shot dead and dozens wounded on Friday when riot police clashed with demonstrators demanding the overthrow of Egyptian President Mohamed Mursi.
Youths threw petrol bombs and shot fireworks at the outer wall of Mursi's Cairo presidential compound as night fell. Police responded by firing water cannon and teargas leading to skirmishes in the surrounding streets.
Two witnesses said they had seen a protester shot dead in Cairo with live ammunition in front of them.
"It's verified. I am at the morgue. He was shot with two bullets, and that's the report of the hospital. The shots were in the neck and the right side of the chest," said one of the witnesses, lawyer Ragia Omran. Medical and security sources confirmed Mohamed Hussein Qurany, 23, was killed with live bullets.
The head of Egypt's ambulance service said at least 54 people had been wounded across the country, mostly in Cairo.
The renewed violence brought an end to a few days of calm after the deadliest week of Mursi's seven months in power.
Protests marking the second anniversary of the uprising that toppled autocrat Hosni Mubarak have killed nearly 60 people since January 25, prompting the head of the army to warn this week that the state was on the verge of collapse.
The head of the Republican Guard, which protects the palace, condemned what he described as attempts to climb the compound walls and storm a gate. In a statement to the state news agency, he urged protesters to keep their demonstration peaceful.
With multi-colored fireworks bouncing off their shields and bursting among them, helmeted and baton-wielding riot police chased protesters at the palace and set their tents ablaze. Petrol bombs briefly set fire to a building inside the compound.
Footage aired by al-Hayat private television showed half a dozen helmeted riot police surrounding a prone man, beating him with truncheons as he lay on the ground beside an armored vehicle near the palace.
Activists quickly posted the pictures online with comments including "As if the revolution never happened".
The protesters accuse Mursi of betraying the spirit of the revolution by concentrating too much power in his own hands and those of his Muslim Brotherhood. The Brotherhood accuses the opposition of trying to overthrow the first democratically elected leader in Egypt's 5,000-year history.
Mohamed Ahmed, 26, protesting at the presidential palace, said: "I am here because I want my rights, the ones the revolution called for and which were never achieved."
There were also scuffles earlier near Cairo's central Tahrir Square, where police fired teargas at stone-throwing youths. In Alexandria, protesters blocked roads, staged a sit-in on the railway and tried to break into the TV and radio building.
Earlier, men dressed in mourning black marched through the Suez Canal city of Port Said, scene of the worst bloodshed of the past eight days, chanting and shaking their fists.
"There is no God but God and Mohamed Mursi is the enemy of God," they chanted. Brandishing portraits of those killed in recent days, they shouted: "We will die like they did, to get justice!"
For the Port Said marchers, Friday was also the first anniversary of a soccer stadium riot that killed 70 people last year. Death sentences handed down on Saturday against 21 Port Said men over the riots helped fuel the past week's violence there, which saw dozens shot dead in clashes with police.
Friday's marches took place despite an intervention by Ahmed al-Tayyeb, head of the 1,000-year-old al-Azhar university and mosque, who hauled in politicians for crisis talks on Thursday where they signed a charter disavowing violence. Mursi's foes said the pact did not require them to call off demonstrations.
"We brought down the Mubarak regime with a peaceful revolution and are determined to realize the same goals in the same way, regardless of the sacrifices or the barbaric oppression," tweeted Mohamed ElBaradei, a former head of the U.N. nuclear watchdog who has become a secularist leader.
The main opposition National Salvation Front denied it was to blame for the demonstrations turning violent. Mursi's office said it would "hold the political forces that may have participated in incitement fully politically responsible, pending results of investigation".
Tahrir Square, ground zero of the uprising against Mubarak, has become a graffiti-scarred monument to Egypt's perpetual turmoil, strewn with barbed wire and burnt-out cars. Vendors sold flag bracelets, pharaonic statues, sunflower seeds, water and fruit while the protesters gathered.
A man with a microphone shouted to the crowd, calling for Mursi to be put on trial. "We came here to get rid of Mursi," said furniture dealer Mohammed al-Nourashi, 57.
The rise of an elected Islamist after nearly 60 years of rule by secular military men in the most populous Arab state is the most important change achieved by two years of Arab revolts.
But seven months since his narrow election victory over an ex-Air Force commander, Mursi has failed to unite Egyptians and protests have made the country seem all but ungovernable. The turmoil has worsened an economic crisis, forcing Cairo to drain its currency reserves to prop up its pound.
Brotherhood leader Mohamed Badie, on his Facebook page, blamed the unrest on "regional and international forces which aim for instability and to stir up problems and ignite strife to damage Egypt ... to thwart the democratic transition".
Brotherhood followers have clashed with demonstrators in the past, especially at the presidential palace which they regard as a symbol of his legitimacy. But the group has kept its men off the streets during the latest violence.
It is far from clear that opposition politicians could call off the street demonstrations, even if they wanted to.
"You have groups who clearly just want a confrontation with the state - straightforward anarchy; you've got people who supported the original ideals of the revolution and feel those ideals have been betrayed," said a diplomat. "And then you have elements of the old regime who have it in their interests to foster insecurity and instability. It is an unhealthy alliance."
Many Egyptians are fed up.
"We are exhausted. This protests thing is a political game whose price the people are paying. I hate them all - liberals and Brotherhood," said Abdel Halim Adel, 60, near the presidential palace.