Egypt’s interim leaders have promised fresh elections by early next year, an offer rejected by supporters of ousted president Mohamed Morsi.
Morsi’s Muslim Brotherhood, which has led demonstrations against last week’s military overthrow of the Islamist leader, has rejected the timetable laid out by interim president Adly Mansour.
The BBC cited senior Brotherhood figure Essam al-Erian as saying the plan for constitutional changes and a vote next year would bring the country "back to square one."
The Brotherhood has called for an "uprising" after the deaths of dozens of its supporters in Cairo on Monday, accusing the military of "massacring" pro-Morsi demonstrators during dawn prayers outside the barracks where it is believed the ex-president is being held.
At least 51 people were reported killed and more than 300 wounded when Egyptian soldiers and police clashed with Islamists there early Monday.
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Mansour later issued a statement expressing "deep sorrow" over the deaths and calling for restraint.
The Brotherhood, meantime, released the names of 42 people killed in the incident.
The interior ministry and military said two policemen and a soldier were killed and blamed "terrorists."
According to the Associated Press, Army Col. Ahmed Mohammed Ali told a news conference that police and troops came under "heavy gunfire" at around 4 a.m., while attackers threw Molotov cocktails from rooftops.
The rooftop attacks were reportedly filmed by Egyptian TV and the video then handed to the military. The video also showed armed protesters firing at close range at the troops; however, it did not show the military reaction.
The United States has called on the Egyptian army to exercise "maximum restraint," but also condemned Brotherhood calls to violence.
Mansour's decree sets a timetable for a referendum on an amended constitution and then for parliamentary elections by February at the latest, to drag the Arab world's biggest country from a crisis that many are warning could devolve into civil war.
Reuters suggested that the decree contained "controversial language" defining the principles of Islamic sharia law, "in what appeared to be an olive branch to Islamists."
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