Egyptian riot police have fired tear gas at thousands of protesters in the streets outside the presidential palace.
President Mohamed Morsi reportedly left the building as the protesters - who are demonstrating against sweeping new powers he granted himself and a new constitution that was rushed through by his supporters - broke through police lines.
The tear gas was fired after a number of people cut through barbed wire erected a few hundred metres from the palace. The police then retreated allowing the protesters to reach the palace walls.
Some of the demonstrators attempted to climb the walls but were stopped by others.
The brief outburst of violence left 18 people injured, none seriously, according to the official MENA news agency.
Up to 100,000 people, largely supporters of liberal and left-wing parties, had gathered near the palace in what they called a "last warning" to the Islamist president.
"The people want the downfall of the regime," some demonstrators chanted in an echo of the mass demonstrations that led to the end of Hosni Mubarak's two decade rule over the country.
Thousands of people waved Egyptian flags and dennouned Mr Morsi's Muslim Brotherhood for having "sold the revolution".
In the central province of Minya, clashes flared between opponents and supporters of Mr Morsi outside the headquarters of the Freedom and Justice Party, the political arm of the Brotherhood.
Anti-Morsi protests also erupted in the Mediterranean city of Alexandria and the central province of Sohag.
Tens of thousands of people also joined protesters in Cairo's Tahrir Square, where several hundred people have been camped out for nearly two weeks.
The November 22 decree issued by Mr Morsi, which expanded his powers and enabled him to put to the draft constitution to a referendum on December 15, has sparked a series of protests and strikes.
Mona Shukri, a Christian protester, said she would boycott the vote.
"The new constitution is like the old one with minor changes. It has no guarantees for the rights of Christians," she said.
Bassam Ali Mohammed, an Islamic law professor, who was also demonstrating near the palace, said: "Egypt is a country where all religions should live together. I love God's law and sharia (Islamic law) but I will vote against the constitution because it has split the people."
Mr Morsi, who took office in June, has insisted that his decree and the new constitution will bring stability to the troubled transition following the 2011 uprising. He said that he protected himself from judicial oversight in order to prevent interference by supporters of Mubarak's regime.
A statement by his office said that on Tuesday he met his deputy, prime minister and several top Cabinet members to discuss preparations for the referendum.