Police have fired tear gas in clashes with tens of thousands of protesters gathered near the presidential palace in the Egyptian capital Cairo.
Some of the protesters cut through barbed wire around the palace,according to BBC.
Crowds have been gathering to protest against what they say is the rushed drafting of a new constitution and the decision by President Mohammed Morsi to extend his powers.
Morsi was in the palace but left as the crowds grew, sources there said.
Many of those gathered outside chanted slogans similar to those directed against the regime of former president Hosni Mubarak during protests in February 2011.
Tear gas was fired after protesters managed to breach a barbed wire cordon surrounding the palace, correspondents say.
But the police quickly retreated, allowing protesters to get closer to the palace walls.
Eighteen people were injured in the brief burst of violence but none seriously, the official Mena news agency reported.
In a statement read out on television, Egyptian security forces called for calm among the protesters.
As well as the protest outside the presidential palace in the suburb of Heliopolis, thousands of demonstrators have gathered in Cairo’s Tahrir Square.
A sizeable crowd has also turned out in Egypt’s second city Alexandria.
Several newspapers refused to go to press on Tuesday in protest at what they say is the lack of press freedom in the constitution.
Morsi adopted sweeping new powers in a decree on 22 November, and stripped the judiciary of any power to challenge his decisions.
He has also called a nationwide referendum for 15 December on a new constitution, which opponents say has been rushed through and fails to protect the rights of minorities, particularly women.
Morsi, who narrowly won Egypt’s first free presidential election in June, says he will give up his new powers once a new constitution is ratified.
His actions have brought out thousands, both his supporters and his opponents, in recent days.
M.D