Egyptian protesters broke through a barbed wire barricade keeping them from the presidential palace in Cairo and some climbed onto army tanks and waved flags.
Up to 10,000 protesters had been penned behind the barrier, guarded by tanks that were deployed yesterday.
The deployment of tanks was due to violence between supporters and opponents of the Islamist president, Mohamed Mursi, in which seven people were killed the night before.
Demonstrators cut the barbed wire and hundreds swarmed through and surged up to the walls of the palace, some kissing the police and military guards surrounding it. "Peaceful, peaceful," they chanted.
Troops of the Republican Guard, which had ordered rival demonstrators to leave the vicinity yesterday, moved to the front gate to secure the main entrance to the palace.
Egyptian police battled thousands of protesters outside President Mohamed Mursi's palace in Cairo, prompting him to leave the building.
Officers fired teargas at up to 10,000 demonstrators angered by Mr Mursi's drive to hold a referendum on a new constitution on 15 December.
Some broke through police lines around his palace and protested next to the perimeter wall.
The crowds gathered in what organisers dubbed "last warning" protests against Mr Mursi, who angered opponents with a decree that expanded his powers.
"The people want the downfall of the regime," the demonstrators chanted.
Mr Mursi ignited a storm of unrest in his bid to prevent a judiciary still packed with appointees of ousted predecessor Hosni Mubarak from derailing a troubled political transition.
Facing the gravest crisis of his six-month-old tenure, the Islamist president has shown no sign of buckling under pressure.
Riot police at the palace faced off against activists chanting "leave, leave" and holding Egyptian flags with "no to the constitution" written on them.
Protesters had assembled near mosques in northern Cairo before marching towards the palace.
The Health Ministry said 18 people had been injured in clashes next to the palace, according to the state news agency.
Despite the latest protests, there has been only a limited response to opposition calls for a mass campaign of civil disobedience.
A few hundred protesters gathered earlier near Mr Mursi's house in a suburb east of Cairo, chanting slogans against his decree and against the Muslim Brotherhood, from which the president emerged to win a free election in June.
Police closed the road to stop them from coming any closer, a security official said.
Opposition groups have accused Mr Mursi of making a dictatorial power grab to push through a constitution drafted by an assembly dominated by his supporters.
They say the draft constitution does not reflect the interests of Egypt's liberals and other groups, an accusation dismissed by Islamists who insist it is a balanced document.
Egypt's most widely-read independent newspapers did not publish today in protest at Mr Mursi's "dictatorship". Banks closed early to let staff go home safely in case of trouble.