Thousands of people are on the streets of the Egyptian capital Cairo for rival demonstrations involving supporters and opponents of President Mohammed Mursi.
The opposition wants Mr Mursi to scrap Saturday's referendum on a new constitution while his supporters say it should go ahead.
Four separate opposition protest marches were due to converge on the presidential palace, which has been walled off with concrete blocks and ringed with tanks.
Anti-government protesters have broken through a barrier protecting the presidential palace but there has been no violence.
A BBC correspondent said the presidential guard appeared to have let the demonstrators through, though the military is allowed to arrest civilians. Several hundred were in the palace compound.
Islamist protesters who support President Mursi and the draft constitution were gathering around Tahrir Square.
Protests had also been planned in the cities of Alexandria and Assiut.