CAIRO (AP) -- Egyptian riot police fired tear gas and clashed all day Thursday with dozens of protesters as they tried to tear down a cement wall built to prevent demonstrators from reaching the parliament and the Cabinet building in central Cairo. Some in the crowd threw rocks and Molotov cocktails.
The violence came on the eve of the second anniversary of Egypt's uprising that toppled longtime authoritarian President Hosni Mubarak. Three weeks of mass protests that erupted on Jan. 25, 2011, eventually forced Mubarak out of office.
Since then, Egypt has undergone a tumultuous transition under the interim leadership of military generals until the election last June of Mohammed Morsi of the Muslim Brotherhood group. His first six months in office were marked by political tensions, street clashes, and an economic crunch that sapped his popularity.
Thursday's clashes, which left dozens injured, may foreshadow a violent anniversary on Friday, when youth activists and opposition groups have called for large rallies in Cairo's Tahrir Square and in front of the presidential palace in the upscale Heliopolis suburb.
Die-hard fans of Egypt's most popular soccer team, Ahly, took part in the clashes, warned in a statement, "the price of blood is blood," in reference to the deaths of many of their friends last year in a violent rampage at a soccer game that left 74 dead.
The group is also calling for mass protests on Jan. 26, the day a court is expected to rule in a trial of security officials related to the soccer deaths, one of the world's bloodiest instances of violence at a sports event.
On Tuesday, in an attempt to assuage anger, Morsi announced that the victims will be considered "martyrs" which meant that their families will receive compensations like those killed in the uprising against Mubarak.
The same day, Egypt's prosecutor general, a Morsi-appointee, asked the court to give more time for the prosecutor to introduce new findings and new defendants before issuing its verdict, in what was seen as another move to postpone the verdict and avoid street violence by the soccer fans, known as Ultras.
On Wednesday, Ultras held a sit-in in front of Egypt's stock market, briefly blocked a highway, and set up tents in Tahrir Square. The group has been long been at odds with police, and it played a key role in anti-Mubarak uprising.
In addition to Ultras, a previously unknown group calling itself the Black Bloc appeared in a video clip posted on social networking sites. Wearing black masks and waving black banners, it warned the Muslim Brotherhood of using its "military wing" to put down protests, saying that if it did it would "go down to the streets and never come back."
In another online statement, the Black Bloc claimed responsibility for attacks on the offices of the Muslim Brotherhood and a fast food chain known to be owned by the group. Egyptian daily Al-Masry al-Youm reported that members of the group appeared in Thursday's clashes.
The opposition has demanded a suspension or radical changes to be made Egypt's newly adopted constitution, which an Islamist-dominated constituent assembly drafted amid deep polarization and mass street protests.
The constitution, which many Egyptians see as a detrimental to civil liberties and a precursor to a religious state, passed with a 64 percent "yes" vote in a December referendum in which around 33 percent of voters took part.
In an online video message posted on Thursday, the nation's most prominent opposition leader Mohamed ElBaradei urged Egyptians to rally in the streets but warned that change will take time.
"I demand from each one of you, all across Egypt, to prove that the revolution must continue and much be completed," said ElBaradei, a Nobel Peace Prize laureate and former head of the U.N. nuclear agency in the message.
"Egyptians rose up for the sake of freedom, dignity and social justice," he said. "We must not stop until we see all the demands achieved. It will take time but we have to put ourselves on the right path."
The opposition wants to use the occasion to put pressure on Morsi and the Muslim Brotherhood, who secular and liberal Egyptians accuse of trying to monopolize power.
Meanwhile, the Brotherhood and other Islamist parties announced they will stay away from the streets on Friday's anniversary and warned the opposition against instigating violence. However, a Brotherhood member said that orders were given for supporters to rally at a mosque located near the presidency. He spoke on condition of anonymity because of the sensitivity of the issue.