A coalition including the Muslim Brotherhood and other Egyptian Islamist parties have rejected opposition demands to delay a December 15 referendum on a controversial draft constitution.

A senior member of the Muslim Brotherhood, Khairat al-Shater, said Saturday the referendum must proceed on the scheduled date without modification or delay.

The announcement comes as the political crisis in Egypt is entering its third week.

In protests over the past two weeks, opponents of Egyptian President Mohamed Morsi have clashed with his supporters over a decree granting the president extraordinary powers and over the planned December 15 referendum.

Mr. Morsi called for what he described as a “comprehensive dialogue” Saturday with the opposition. But opposition leaders say talks with Mr. Morsi's government cannot begin until the decree is revoked and the referendum is canceled.

Egypt's military said Saturday it supports dialogue to resolve political differences.

On Friday, Egypt postponed the start of early voting on the referendum by Egyptians abroad. Some analysts said the move could signal that the president may be willing to negotiate with the opposition.

Opposition leader Mohamed ElBaradei said the president's refusal to compromise has created what ElBaradei calls “a disaster.”

Two protesters were killed in clashes this week, and nearly 700 people have been injured since the start of the demonstrations. Mr. Morsi has said he will not tolerate killings or sabotage.