The streets around Cairo's Tahrir Square were again roiled by violent clashes between police and protesters Friday, as crowds gathered to mark two years since the start of the revolution that ousted President Hosni Mubarak.

One pocket of violence broke out a few blocks from the square, where police erected a barrier of concrete blocks on a street leading to the Interior Ministry and other government buildings.

Young protesters threw rocks over the barrier at officers stationed there, who responded sporadically with tear gas or threw stones themselves.

At least 29 protesters were treated for cuts, broken bones and birdshot injuries, Health Ministry spokesman Khaled El Khatib said. Six police officers were also hurt in the disorder near Tahrir Square, the Interior Ministry said.

Egyptian police also fired tear gas to disperse protesters who tried to cross barbed wire outside the presidential palace, to the northeast of the city, according to state-run Nile TV.

And clashes broke out between opponents and supporters of President Mohamed Morsy in the coastal city of Alexandria, according to multiple eyewitnesses who spoke to CNN.

In the city of Ismailiya the main office of the Freedom and Justice Party, the political wing of the Muslim Brotherhood, which backs Morsy, was torched by protesters, state TV reported.

The numbers making their way toward Tahrir Square grew through the day, swelling to an estimated 10,000 by the afternoon.

While some were celebrating the anniversary, many of the groups in the square Friday were made up of liberals, moderates and secularists who claim that Morsy and the Muslim Brotherhood have hijacked the revolution and its ideals.

Morsy has said the protests are unfair and that he is upholding the democratic principles of the revolution, but the country's divisions continue to fester.

On January 25, 2011, massive crowds gathered in Tahrir Square to protest Mubarak's regime, resulting in his eventual ouster and two years of upheaval.

As many as 20,000 protesters spilled into the streets in an unprecedented display of anti-government rage on that first day. At one point, cheering protesters in the world's largest Arab nation broke through riot police who had lined Tahrir, or Liberation, Square. Police lobbed tear gas and used water cannon but the protesters were undaunted.

Since then, the square has continued to be a focal point for demonstrations for Egyptians from all factions.

Journalist Ramy Francis and CNN's Reza Sayah reported from Cairo; Laura Smith-Spark wrote in London.