Huge demonstrations were expected in Egypt Friday on the second anniversary of the revolution that ousted Hosni Mubarak and brought in an Islamist government, as political tensions simmer and economic woes bite.
Demonstrators were making their way to Cairo's Tahrir Square, where some had already spent the night, after police clashed Thursday with protesters who tried to dismantle a wall of concrete blocks closing a street leading to the square.
Some demonstrators hurled rocks at riot police who responded with tear gas grenades. The interior ministry said five policemen were injured and appealed to demonstrators to avoid confrontation with the security forces.
The secular-leaning opposition has called for mass protests against President Mohamed Morsi and the Muslim Brotherhood from which he hails, using the same slogan that brought Egypt to its feet in 2011: "Bread, freedom, social justice."
"Go out into the squares to finally achieve the objectives of the revolution," opposition leader and former head of the UN's atomic agency Mohamed ElBaradei wrote on his Twitter account.
Friday's demonstrators were set to gather at Tahrir Square, iconic birthplace of the 2011 revolution, and in front of the presidential palace, where anti-Morsi rallies last December erupted into deadly clashes with Islamist supporters.
Rallies were also due to be staged elsewhere, notably in Egypt's second city Alexandria, and the authorities have prepared for a heavy security presence, an official said.
The Muslim Brotherhood has not officially called for its own rallies, instead marking the anniversary by launching a charitable and social initiative dubbed "Together we will build Egypt."
Morsi urged Egyptians to spend the anniversary in a "peaceful and civilised way," in a speech on Thursday to mark the birthday of the Prophet Mohammed.
But the threat of violence remained, ahead of a court verdict due on Saturday in the trial of dozens of defendants over the worst football disaster in Egyptian history.
More than 70 people were killed in Port Said in February last year during clashes in the Suez Canal city between fans of home side Al-Masry and diehard supporters Cairo's Al-Ahly, known as Ultras.
Al-Ahly supporters warned of violent protests and a "new revolution" if Saturday's verdict goes against them.
After the seismic political changes of 2011, the Arab world's most populous nation is struggling to find a balance between a leadership that boasts the legitimacy of the ballot box and opponents who accuse the Islamists of betraying the goals of the revolution that brought them to power.
Egypt is also in the throes of an economic crisis as foreign investment and tourism revenues dwindle, the Egyptian pound stands at its lowest level against the dollar and a budget deficit shows no sign of recovery.