Police fired teargas to disperse protesters who tried to cross barbed-wire barriers guarding Egypt's presidential palace on Friday, state television reported, during violent demonstrations against President Mohamed Morsi.
The state-run Nile News TV station gave no further details in an urgent headline flashed on the screen.
Earlier, hundreds of youths fought Egyptian police in Cairo on the second anniversary of the revolt that toppled Hosni Mubarak and brought the election of an Islamist president who protesters accuse of riding roughshod over the new democracy.
The Jan. 25 anniversary showcased the divide between the Islamists and secular foes hindering President Mohamed Morsi's efforts to get a stagnant economy moving, and reverse a plunge in Egypt's currency, by attracting back investors and tourists.
Inspired by Tunisia's ground-breaking popular uprising, Egypt's revolution spurred further revolts across the Arab world. But the sense of common purpose that united Egyptians two years ago has given way to internal strife that has only worsened and last month triggered lethal street battles.
Opponents of Morsi and his Muslim Brotherhood allies began massing in Cairo's Tahrir Square to revive the demands of a revolution they say has been betrayed by Islamists.
Police battled hardcore protesters who threw petrol bombs and firecrackers as they tried to approach a wall blocking access to government buildings near the square in the early hours of the morning.
Clouds of tear gas fired by police filled the air. At one point, riot police used one of the incendiaries thrown at them to set ablaze at least two tents erected by the youths, a Reuters witness said. Clashes between stone-throwing youths and the police continued in streets near the square into the day.
Ambulances ferried away a steady stream of casualties. The health ministry said 25 people had been injured since Thursday in clashes around Tahrir Square.
Some protesters pledged to march to Morsi's palace.
Thousands more protested against the Brotherhood in cities across Egypt including Suez, Ismailia, Port Said and Alexandria.
The Brotherhood decided against mobilising in the street for the anniversary, wary of the scope for more conflict after violence in December that was fuelled by Morsi's campaign to fast-track an Islamist-tinged constitution.
"The people want to bring down the regime," declared banners in the square, echoing the main Arab Spring slogan of 2011. "Save Egypt from the rule of the Supreme Guide," said another, a reference to leader of the Muslim Brotherhood, Mohamed Badie.
Morsi, in a speech on Thursday marking the Prophet Mohammad's birthday, called on Egyptians to mark the anniversary "in a civilised, peaceful way that safeguards our nation, our institutions, our lives".
"The Brotherhood is very concerned about escalation, that's why they have tried to dial down their role on Jan. 25," said Shadi Hamid director of research at the Brookings Doha Center.
"There may very well be the kinds of clashes that we've seen before, but I don't see anything major happening that is going to fundamentally change the political situation," he said.
With its eye firmly on forthcoming parliamentary elections, the Brotherhood marked the anniversary with a big charity drive across the nation. It plans to deliver medical aid to 1 million people and distribute affordable basic foodstuffs.
Writing in Al-Ahram, Egypt's flagship state-run daily, Brotherhood leader Badie said the country was in need of "practical, serious competition" to reform the corrupt state left by the Mubarak era.
"The differences of opinion and vision that Egypt is passing through is a characteristic at the core of transitions from dictatorship to democracy, and clearly expresses the variety of Egyptian culture," he wrote.
Still, Morsi faces discontent on multiple fronts.
His opponents say he and his group are seeking to dominate the post-Mubarak order. They accuse him of showing some of the autocratic impulses of the deposed leader by, for example, driving through the new constitution last month.
"I am taking part in today's marches to reject the warped constitution, the 'Brotherhoodisation' of the state, the attack on the rule of law, and the disregard of the president and his government for the demands for social justice," Amr Hamzawy, a prominent liberal politician, wrote on his Twitter feed.
The Brotherhood dismisses many of the criticisms as unfair. It accuses its opponents of failing to respect the rules of the new democracy that put the Islamists in the driving seat by winning free elections.
Six months into office, Morsi is also being held responsible for an economic crisis caused by two years of turmoil. The Egyptian pound has sunk to record lows against the dollar.
Other sources of friction abound. Little has been done to reform brutal Mubarak-era security agencies. A spate of transport disasters on roads and railways neglected for years is feeding discontent as well. Activists are impatient for justice for the victims of violence over the last two years.
These include hardcore soccer fans, or ultras, who have been rallying in recent days to press for justice for 74 people killed in a soccer stadium disaster a year ago in Port Said after a match between local side al-Masry and Cairo's Al Ahly.
A verdict in the case brought against 73 people charged in connection with the deaths had been expected on Saturday, but could be delayed after a request by the prosecutor for time to present new evidence. A delay will likely kindle more protests.
The parties that called for Friday's protests listed demands including a complete overhaul of the constitution.
Critics say the constitution, which was approved in a referendum, offers inadequate protection for human rights, grants the president too many privileges and fails to curb the power of a military establishment supreme in the Mubarak era.
Morsi's supporters say enacting the constitution quickly was crucial to restoring stability desperately needed for economic recovery, and that the opposition is making the situation worse by perpetuating unrest.