Police on Friday battled protesters who threw petrol bombs and firecrackers as they tried to approach a wall blocking access to government buildings near the square in the pre-dawn hours.
The January 25 anniversary showcased the divide between the religious and their secular foes that is hindering President Mohamed Mursi's efforts to revive an economy in crisis and reverse a plunge in Egypt's currency by enticing back investors and tourists.
Inspired by Tunisia's historic 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 Mursi and his Muslim Brotherhood allies massed in Cairo's Tahrir Square to revive the demands of a revolution they say has been betrayed by Islamists.
"Our revolution is continuing. We reject the domination of any party over this state. We say no to the Brotherhood state," Hamdeen Sabahy, a popular leftist leader, told media as he made his way to the square for the rally.
Clouds of teargas 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.
There were similar scenes in Alexandria, where protesters and riot police skirmished near local government offices. Tear gas fouled the air and black smoke billowed from tyres set ablaze by youths. Nine people were wounded by birdshot pellets, according to medical and security sources.
Some protesters pledged to march to Mursi's Cairo palace.
Thousands more protested against the Brotherhood in cities across Egypt including Suez, Ismailia and Port Said.
The Brotherhood decided against mobilising in the street for the anniversary, wary of the scope for more conflict after violence in December that was stoked by Mursi's decision to fast-track an extremist-tinged constitution.
"The people want to bring down the regime," chanted crowds in Tahrir Square, where numbers stood at several thousand by early afternoon. "Save Egypt from the rule of the Supreme Guide," said another, a reference to leader of the Muslim Brotherhood, Mohammed Badie.
Prophet Muhammad's birthday
"We are not here to celebrate but to force those in power to submit to the will of the people. Egypt now must never be like Egypt during Mubarak's rule," said Mohamed Fahmy, an activist.
Mursi, in a speech on Thursday marking the Prophet Muhammad'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 January 25," said Shadi Hamid, director of research at the Brookings Doha Centre.
"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.
'Practical, serious competition'
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 one-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, Mursi 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 government in the driving seat by winning free elections.
Six months into office, Mursi 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. – Reuters