Huge crowds were expected to demonstrate in Egypt on Friday for the second anniversary of the revolution that ousted Hosni Mubarak and brought in an Islamist government, at a time of political tensions and economic woes.
The secular-leaning opposition called for mass street 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.
Tensions ran high on Thursday as police clashed with protesters who tried to dismantle a wall of concrete blocks shutting off a street leading to Cairo's Tahrir Square, to ease the movement of demonstrators.
The demonstrators were to gather at Tahrir Square, iconic birthplace of the 2011 revolution, and in front of the presidential palace, where anti-Morsi rallies last December descended into deadly clashes with Islamist supporters.
Rallies elsewhere in the country, notably second city Alexandria, were also due to take place, and security forces have prepared for a heavy deployment, an official said.
The Muslim Brotherhood has not officially called for its own rallies, marking the anniversary instead by launching a charitable and social initiative dubbed "Together we will build Egypt."
Morsi urged Egyptians to mark the revolution's anniversary in a "peaceful and civilised way," in a speech on Thursday to mark the birthday of the Muslim Prophet Mohammed.
The threat of violence, however, remained, with a court verdict due on Saturday in the trial of dozens of defendants over the country's worst football disaster.
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 the Ultras of Cairo's Al-Ahly.
Ahly supporters warned of violent protests and a "new revolution" should the verdict not rule in their favour.
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 and opponents who accuse the Islamists of betraying the goals of the revolution that brought them to power.
The country also faces an economic crisis, as foreign investment and tourism dwindle, the Egyptian pound stands is at its lowest level against the dollar and a budget deficit shows no sign of recovery.