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Egypt's stock index fell by nearly 10 percent Sunday in the first trading since President Mohammed Morsi issued decrees to assume sweeping new powers, while a 15-year-old boy was killed in an attack on the Muslim Brotherhood's headquarters and police in central Cairo fired tear gas at protesters.

Morsi's edicts, which were announced on Thursday, place him above oversight of any kind, including that of the courts. The move has thrown Egypt's already troubled transition to democracy into further turmoil, sparking angry protests across the country to demand the decrees be immediately rescinded.

The judiciary, which was the main target of Morsi's edicts, has pushed back. Judges and prosecutors stayed away from several courts in Cairo and across much of the country.

The nation's highest judicial body called on judges and prosecutors to return to work and announced that its members would meet with Morsi on Monday to try to persuade him to restrict immunity to major state decisions like declaring war or martial law.

With the opening bell of the country's stock market on Sunday, the first day of the workweek in Egypt, the turmoil spread from the country's bitter politics to its already ailing economy. The Egyptian Exchange's EGX30 index dropped 9.59 percentage points, making the losses among the biggest since the turbulent days and weeks immediately after the ouster in a popular uprising of authoritarian leader Hosni Mubarak last year.

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The loss in the value of shares was estimated at close to $5 billion.

Morsi, who narrowly won the presidency in June, said his measures were 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.

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.

Prominent Egyptian democracy advocate Mohammed ElBaradei 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.

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 Damanhour, 93 miles Northwest of Cairo, a 15-year-old boy died in attacks on the Muslim Brotherhood's headquarters. Forty people were injured in the violence, according to security officials who spoke to the Associated Press. They spoke on condition of anonymity because they were not authorized to brief reporters.

Suspected militants also 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.

The Associated Press contributed to this report.