At least 18 people have been killed in anti-government protests in Egypt on the anniversary of the 2011 uprising that toppled Hosni Mubarak, security sources say.
Three police cadets were among the dead, and dozens of protesters were also injured, the officials said.
The clashes follow the death of an activist in a march in the capital Cairo on Saturday.
The protests were staged to mark the fourth anniversary of Egypt's 2011 uprising, which toppled long-time leader Hosni Mubarak.
Security in major cities was tightened ahead of the anniversary, and key locations in Cairo were blocked off.
Dozens of people were killed in similar protests last year.
In the bloodiest day of protests since Abdel Fattah al-Sisi was elected president in June, security forces and plain-clothed police fired at protesters, witnesses said.
The anniversary is a test of whether Islamists and liberal activists have the resolve to challenge a government that has stamped out dissent since the then-army chief ousted elected Islamist president Mohamed Morsi in July 2013 after mass protests against his rule.
After nightfall, gunfire and sirens could be heard in central Cairo as armoured personnel carriers moved through the city centre.
Gunmen in a car opened fire on a security checkpoint near the Pyramids, killing two policemen, security sources said.
Protesters set fire to a government building on a street near the Pyramids, state media said.
Dozens of protesters were killed during last year's anniversary. Again this year, security forces fanned out across the capital and other cities.
The heaviest death toll was in the Cairo suburb of Matariya, a Muslim Brotherhood stronghold.
Special forces fired pistols and rifles at protesters, a Reuters witness said. Eight people, including one policeman, were killed, according to the Health Ministry.
People in Matariya chanted "down with military rule" and "a revolution all over again". Demonstrators threw Molotov cocktails at security forces and fires raged.
Riot police backed by soldiers in armoured vehicles sealed off roads, including those leading to Cairo's Tahrir Square, the symbolic heart of the 2011 revolt.
In downtown Cairo, riot police with rifles and plain clothed men with pistols chased protesters through the streets.
Six people were killed in separate protests in Alexandria, Egypt's second biggest city, Giza governorate outside of Cairo and the Nile Delta province of Baheira, security sources said.
A bomb wounded two policemen stationed outside a Cairo sports club, the sources said.
Signs of discontent built up as the anniversary of the revolt against Mr Mubarak approached, and a liberal woman activist, Shaimaa Sabbagh, was killed at a protest on Saturday (local time).
About 1000 people marched in her funeral procession on Sunday. The Health Ministry said she had been shot in the face and back and Interior Ministry spokesman Hany Abdel Latif said an investigation into her death had begun, adding: "No one is above the law."
Mr Sisi's crackdown has neutralised the Muslim Brotherhood but failed to end an Islamist insurgency in the Sinai Peninsula near the Israeli border.
A curfew imposed in north Sinai was extended for three months, authorities said. Islamist militants based in the Sinai have killed hundreds of police and soldiers since Mr Morsi's removal. They have pledged support for Islamic State, the ultra-hardline group that seized parts of Iraq and Syria.
After four years of political and economic turmoil following Mr Mubarak's fall, many Egyptians have overlooked allegations of widespread human rights abuses and praised Mr Sisi for restoring a measure of stability.
Mr Sisi, who served as military intelligence chief under Mr Mubarak, has also taken bold steps to repair the economy, such as cutting costly fuel subsidies.
But his critics accuse him of restoring authoritarian rule and repealing freedoms won in the uprising that ended three decades of iron-fisted rule under Mr Mubarak.
In a televised address on Saturday, Mr Sisi praised the desire for change that Egyptians showed four years ago but said it would take patience to achieve all of "the revolution's goals".
Sheikh Youssef al-Qaradawi, an Egyptian-born cleric based in Qatar who supports the Brotherhood, called for protests on Sunday and said Mr Morsi was Egypt's legitimate leader.
Mr Qaradawi's outspoken support for the Islamist movement has fuelled a diplomatic rift between Qatar and its Gulf Arab allies which, like Cairo, consider the group a security threat.