The 13 parties "insist that the referendum on the constitution take place on the scheduled date (December 15), with no modification or delay," according to a joint statement.
Khairat al-Shater, the number two of the Muslim Brotherhood, read out the statement to the media.
Meanwhile Egypt's military today warned of "disastrous consequences" if the political crisis gripping the country is not resolved through dialogue.
"The Armed Forces watches with sorrow and concern the developments of the current circumstances, and the status of the divisions and unfortunate events threatening the pillars of the Egyptian state and the national security," a military statement said.
The statement urged all political forces to pursue dialogue.
"We support national dialogue, and serious and sincere democratic process on the debatable issues and points, to reach consensus," it said.
"The dialogue approach is the best way and the only access to consensus on the interest of the nation and citizens. The opposite would get us into a dark tunnel with disastrous consequences, which we will not allow," it added.
The statement also said that the loyalty of the Egyptian armed forces is for the people. "The military institution always sides with the great people of Egypt, and is keen on their unity," it said.
An army spokesperson further called on all citizens to maintain the security and safety of the nation and place Egypt's interest above all, in the framework of "legal legitimacy and democratic rules which agreed upon and accepted to move to the future on that basis."
The military statement came as demonstrators fenced off an administrative building in Tahrir Square angry over President Mursi's attempts to push through a new constitution. MORE
