At least one person has been killed in clashes outside an office of Egypt's ruling Muslim Brotherhood, reports say.
The violence at the Freedom and Justice Party headquarters in the Nile Delta town of Damanhour came amid widespread anger in Egypt over President Mohamed Morsi's decision to grant himself sweeping new powers.
"Brotherhood member, Islam Fathy Masoud, 15, was killed and 60 were injured after thugs attacked the Muslim Brotherhood's headquarters in Damanhour in the total absence of police forces," the party's website said.
Mr Morsi had sought to calm the anger over the decree which effectively protects the president's decisions from any legal challenge.
"The presidency reiterates the temporary nature of the said measures, which are not meant to concentrate power, but ... to devolve it to a democratically elected parliament ... as well as as preserving the impartiality of the judiciary and to avoid politicising it," a statement from the president's office said.
The statement also reaffirmed that the new powers - which Mr Morsi says were taken to protect Egypt's revolution from followers of former president Hosni Mubarak - would only apply until a new constitution is adopted.
Mr Morsi will meet senior members of the judiciary, which has changed little since the Mubarak era, on Monday after they called the move an "unprecedented attack" on their authority.
Angry protesters accusing him of blatant power grab clashed with police for a third consecutive day in central Cairo on Sunday as Egypt's stock market reacted with a nearly 10% fall.
More than 500 people are believed to have been injured in violent protests since the decree was announced on Thursday.
Several offices belonging to the Muslim Brotherhood's Freedom and Justice Party have been torched.
Activists are staging a sit-in at Cairo's Tahrir Square - the symbolic hub of the popular uprising that forced Mubarak from power - and a mass protest has been called for Tuesday. The Muslim Brotherhood is planning a counter demonstration.
Many of Morsi's political opponents share the view that Egypt's judiciary needs reform, but they disagree with the concentration of power into the hands of the president.