Egyptian opposition supporters are protesting across the country on the second anniversary of the uprising that swept Hosni Mubarak from power.
Police clashed with President Mohammed Morsi’s opponents in Cairo, dispersing protesters outside his palace as thousands gathered in Tahrir Square.
Alexandria and Suez also saw clashes. In Ismailia, protesters set fire to the HQ of the Muslim Brotherhood’s party.
Critics accuse Mr Morsi of betraying the revolution, which he denies.
The president has appealed for calm to end the clashes, in which more than 100 people have been injured nationwide since Thursday.
Concrete walls
On Friday, police fired tear gas to disperse protesters who had tried to cross barbed-wire barriers outside the presidential palace in Cairo, state TV reported. Protesters’ tents were also dismantled.
Earlier, some protesters erected checkpoints at the entrances to Tahrir Square to verify the identities of people passing through. Others set up an exhibition of photographs of those killed at various 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 Islamist movement to which Mr Morsi belongs.
A protester called Hany Ragy told the BBC: “I voted for Morsi because I did not want to see someone from the last regime in power again. But he has not fulfilled his promises. The economy has crumbled.”
“I am here to put pressure on the government to enact proper reform.”
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.
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.
People are now applying the same chants to Mr Morsi that they did two years ago for Mr Mubarak, our correspondent adds.
There are reports that opposition supporters have blocked railway lines leading both to the north and south out of Cairo.
Smaller rallies are taking place in other cities, including Alexandria, Ismailia, Suez and Port Said.
In Ismailia, witnesses said youths had broken into and ransacked the offices of the Freedom and Justice Party, the political arm of the Muslim Brotherhood, before setting fire to it.
The city’s governorate headquarters was later also stormed.
Clashes were reported in at least two locations in Alexandria, with police firing tear gas and protesters burning tyres. At least 10 people were reportedly injured.
“The smoke is black, there is a lot of gas. There are people on the ground because they can’t breathe,” one demonstrator told AFP.
State TV showed protesters in Suez throwing stones at public buildings as motorcycles carried injured people away. Mena news agency said 12 police were injured in the city.
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 does not protect adequately freedom of expression or religion.
Ahead of Friday’s rally Mohamed ElBaradei, a leading opposition figure, 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.”
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.
Former President Hosni Mubarak is currently in detention at a military hospital. An appeals court recently overturned the 84 year old’s life sentence over the deaths of protesters and ordered a retrial.