Opponents of Egyptian President Mohammed Morsi have stormed an office of his Muslim Brotherhood movement in the country's north, triggering a fight that killed one person and wounded at least 40 others.

Brotherhood leaders identified the fatality as a 15-year-old Islamist youth, but it was not clear how he was killed in Sunday's incident in the Nile Delta town of Damanhour. The teenager is the first person known to be killed since reformist and liberal activists began street protests to denounce Mr. Morsi for granting himself sweeping powers in a decree last Thursday.

Opposition activists camped in Cairo's Tahrir Square for a third day Sunday to demand a reversal of the presidential decree. The protesters blocked traffic and engaged in intermittent battles with police in nearby streets.

Egypt's main stock index slumped nearly 10 percent Sunday as investors gave their first response to Mr. Morsi's move and subsequent protests. It was one of the market's biggest losses since the days and weeks after longtime president Hosni Mubarak was ousted in a popular uprising last year.

In Washington, influential U.S. Senator John McCain criticized Mr. Morsi's decree as “unacceptable,” in an interview with the television network Fox News.

“We thank Mr. Morsi for his efforts in brokering a cease-fire (between Israel and Hamas), which by the way is incredibly fragile, but this (decree) is not acceptable. This is not what U.S. taxpayers expect. Our dollars (should) be directly related to the progress toward democracy which you (Morsi) promised the people of Egypt when your party and you were elected president.”

The Obama administration has proposed a $1 billion debt relief package for Egypt to help revive its struggling economy. Egypt also has received billions of dollars in U.S. military aid over three decades of close relations.

The U.S. State Department said the Morsi declarations “raise concerns for many Egyptians and for the international community.” It said one of the aspirations of the 2011 revolution was “to ensure that power would not be overly concentrated in the hands of any one person or institution.”

In his decree, President Morsi said he was barring courts from challenging his decisions, in a move aimed at speeding up Egypt's democratic transition and bringing accountability to former officials accused of crimes under Mubarak. Critics accused Mr. Morsi of taking on dictatorial powers like those of his predecessor.

Some Egyptian judges observed a strike on Sunday to protest the decree. But Egypt's Supreme Judicial Council urged them to return to work and said it will meet with Mr. Morsi on Monday to try to resolve the dispute. The council of senior judges said it will try to persuade the president to restrict his absolute powers to sovereign matters such as war and peace.

Mr. Morsi's office issued a statement Sunday reiterating what he called the “temporary” nature of his decree, which he said will last until Egypt elects a new parliament under a revised constitution. The president also pledged to hold a dialogue with all political forces in Egypt on the drafting of the new constitution.

A spokesman for Mr. Morsi's ruling Freedom and Justice Party said the decree likely will last for “two months, maybe less.”

“So (to) try to show this (Morsi decree) as a dictatorship is wrong,” said Nader Omran. “He is trying to take care of the country. He tried to secure stability for this country. Securing stability is to have a constitution and afterwards to have a parliament, and after the parliament to have a government. And he can pass the authority to these elected institutions. So that's what he wants to do.”

Reformists and liberals fear the Islamist-dominated assembly revising the charter will produce a document with an Islamist slant.

Mr. Morsi's opponents and supporters have called for rival mass rallies in Cairo Tuesday.