A 15-year-old is dead after rioters stormed a Muslim Brotherhood headquarters building in the clash between protesters and police late Sunday in the town of Damanhoor in the Nile delta, according to Egyptian security officials.
More than 40 others were also injured. Officials spoke on condition of anonymity because they were not authorized to brief reporters.
It was the first death in three days of street battles after a power grab by the country's president.
The demonstrators protested Egyptian President Mohammed Morsi's decree granting himself immunity from judicial review as well as other measures neutralizing the judges.
Egypt's highest judicial body called on judges and prosecutors to return to work and announced that its members would meet with Morsi on Monday after declaring a nationwide strike over the weekend.
The Supreme Judicial Council said late Sunday that it wants to discuss Morsi's decree shielding his decisions from judicial review.
Morsi says his measures are designed to "protect the revolution," but they triggered an uproar among non-Islamist political groups now vowing to press on with street protests to force him to back down.
Reform politician Mohamed el Baradei and more than 20 Egyptian rights groups have called for decrees to be withdrawn.
Critics say the measures threaten the independence of Egypt's judiciary, and the association representing judges in Egypt has called on courts to suspend all but their most vital activities in protest.
Broadening powers is 'temporary'
Late Sunday, Morsi's office issued an English-language statement defending his decrees, repeating the argument he used when addressing supporters Friday outside his Cairo palace that the measures were designed to bolster the country's transition to democratic rule and dismantle Mubarak's old regime.
"The presidency reiterates the temporary nature of the said measures, which are not meant to concentrate powers," it said.
The statement also pledged Morsi's commitment to engaging all political forces in drafting a new constitution. Secular and Christian members withdrew from the panel drafting the document, claiming that the Islamists who dominate the body have hijacked the process to produce a charter with an Islamist slant.
El Baradei warned Saturday of increasing turmoil that could potentially lead to the military stepping in unless Morsi rescinds his new powers, as the country's long fragmented opposition sought to unite and rally new protests.
Egypt's liberal and secular forces — long divided, weakened and uncertain amid the rise of Islamist parties to power — are seeking to rally themselves in response to the decrees.
On Sunday, protesters clashed with police in Cairo's Tahrir square, birthplace of the uprising that toppled Mubarak, and in the side streets and avenues leading off the plaza. The Interior Ministry, which is in charge of the police, said 267 protesters have been arrested and 164 policemen injured since the unrest began a week ago, initially to mark the anniversary of street protests a year ago against the nation's then-military rulers. Forty-two protesters were killed in those demonstrations.
The ministry did not say how many protesters were injured in the latest clashes, but security officials put the figure at 260. The officials spoke on condition of anonymity because they weren't authorized to release the information.
Military installation blows up
Several dozen protesters are staging a sit-in in Tahrir, vowing not to leave before Morsi rescinds his decrees. The two sides have called for massive rival protests on Tuesday at two Cairo locations less than a mile apart, raising the possibility of renewed clashes between supporters and opponents of Morsi.
In the latest sign of the unrest sweeping the country, suspected militants blew up a military installation under construction in the central Sinai Peninsula area of al-Qaseema early Sunday, wounding three workers, according to security officials speaking on condition of anonymity because they were not authorized to brief the media.
It was the second blast to target a military building in Sinai in as many days. On Saturday, another border guard structure under construction was blown up in Rafah, close to the Israeli border.
Nobody claimed responsibility for either attack, but authorities have been battling al Qaeda-inspired militants in Sinai who have stepped up attacks against Egyptian security forces, and even on occasion staged cross-border raids targeting Israelis.
Egypt's stock market plummeted nearly 10 per cent on Sunday, the first day of trading since Morsi's assumption of extra powers.