Egyptian opposition supporters are gathering in central Cairo to mark two years since the start of the uprising that swept Hosni Mubarak from power.
Police clashed with opponents of President Mohammed Morsi who are now converging on Tahrir Square.
There are clashes in Alexandria and Suez. In Ismailia protesters set fire to the Muslim Brotherhood's party HQ.
Critics accuse Mr Morsi of betraying the revolution. He denies this and has appealed for calm.
An appeals court recently overturned Mr Mubarak's life sentence over the deaths of protesters and ordered a retrial.
The 84-year-old former leader remains in detention at a military hospital.
Opposition supporters began converging on Tahrir Square on Friday morning. Some protesters have erected checkpoints to verify the identities of people passing through.
Others have set up an exhibition of photographs of those killed in protests over the past two years.
"Our revolution is continuing. We reject the domination of any party over this state. We say no to the Brotherhood state," leftist leader Hamdeen Sabahi told the Reuters news agency, referring to the Muslim Brotherhood, to which Mr Morsi belongs.
The roads leading from Tahrir Square to several nearby government buildings and foreign embassies have been blocked by concrete walls since last November.
Demonstrators tried to dismantle one of them on Thursday night, but a new wall was built to block entry to the Cabinet headquarters.
The unrest continued overnight. On Friday, Nile TV reported worsening clashes outside the interior ministry near Tahrir Square.
The BBC's Aleem Maqbool in Tahrir Square says there are now large numbers of protesters there, but that the violence is restricted to a small corner of it, where teenagers are throwing stones at the parliament building.
At least 25 people have been injured in clashes since Thursday.
The Muslim Brotherhood has not officially called for its own street rallies. It plans to mark the revolution by launching charitable and social initiatives.
'Bread and freedom'One of the demonstrators at Tahrir Square, Hanna Abu el-Ghar, told the BBC: "We are protesting against the fact that after two years of the revolution, where we asked for bread, freedom and social justice, none of our dreams have come true."
The liberal opposition accuses Mr Morsi of being autocratic and driving through a new constitution that favours Islamists and does not sufficiently protect the rights of women or Christians.
Ahead of the planned rally Mohamed ElBaradei, a leading opposition figure and former head of the UN atomic agency, said is a statement: "I call on everyone to take part and go out to every place in Egypt to show that the revolution must be completed."
Smaller rallies are taking place in other cities, including Alexandria, Ismailia, Suez and Port Said.
Clashes were reported in at least two locations in Alexandria, with police firing tear gas and protesters burning tyres.
"The smoke is black, there is a lot of gas. There are people on the ground because they can't breathe," one demonstrator there told AFP.
State TV showed protesters in Suez throwing stones at public buildings as motorcycles carried injured people away.
The government is also being blamed for a deepening economic crisis.
The president has dismissed the opposition's claims as unfair, instead calling for a national dialogue.
Mr Morsi and his supporters accuse their opponents of undermining democracy by failing to respect the Islamists' victory in elections a year ago.
In a speech on Thursday marking the Prophet Muhammad's birthday, President Morsi called on Egyptians to celebrate the anniversary "in a civilised, peaceful way that safeguards our nation, our institutions, our lives".
Last month, he described the new constitution as "historic" and also said that boosting Egypt's economy was his priority.
He also admitted that mistakes had been made but insisted he would never make a decision except in the interests of the country.