Egyptian riot police have fired tear gas at opponents of Islamist President Muhammad Morsi outside the presidential palace in Cairo as demonstrators broke through police lines.
Demonstrators reportedly cut through barbed wire around the palace, allowing tens of thousands of people to surround the compound.
Presidential sources said Morsi had been in the palace but left as the crowds outside began to grow.
The protest was called against Morsi's assuming sweeping powers and against a draft constitution, due to be put to a vote on December 15.
A woman who took part in the protest outside the palace said the new constitution, if approved, would turn Morsi into a dictator.
"We just want to tell the president that we are here and let him know that we accept that people elected you and we accept you as president, but announcing a constitution that makes you a dictator -- no!" she said.
"A constitution that we don't all agree with, with most of the articles not agreed on by us -- no. And this is our right as the Egyptian people."
Another protester expressed his disagreement with the Muslim Brotherhood, the Islamist political movement from which Morsi himself hails.
"The main reason [of the demonstration] is the corruption of the regime itself; I mean the corruption of the Muslim Brotherhood -- this is the most important reason to come here," the man said.
"They have an ideology of leading the Egyptian people by force, they want to hold on to everything, moreover, they made many pledges to implement all sorts of reforms, but they have delivered nothing yet."
Morsi has angered his opponents in the last month by moving to concentrate power into the post of the presidency by decree.
Protesters accuse him of having "sold the revolution" that toppled longtime strongman Hosni Mubarak last year.
Protests over the past weeks were sparked by a November 22 decree issued by Morsi expanding his powers and enabling him to put to a mid-December referendum a draft constitution.
The draft has become the object of a political and ideological battle between Islamists and the largely secularist opposition. The opposition says the constitution fails to protect the rights of women and minority groups.
Morsi says the constitution has to be approved before he can rescind his decrees.
The draft constitution was hurriedly approved last week by an assembly dominated by Morsi's Islamist allies.
Meanwhile, several independent and opposition newspapers have suspended publication to protest perceived strictures on freedom of expression in the draft constitution.
The December 4 protest involves at least eight publications.
Privately owned TV channels are planning their own protest on December 5, when they say they will cancel all broadcasts.
Some 10,000 protesters are also reported to have gathered in the coastal city of Alexandria and in the province of Sohag.
In the central province of Minya, Morsi's opponents clashed with his supporters outside the local headquarters of Morsi's Freedom and Justice Party, the political arm of the Muslim Brotherhood.
Police fired tear gas to disperse the crowds.
With reporting by AP, AFP, and Reuters