Egypt's Muslim Brotherhood and its allies have called for a referendum on a controversial constitution to go ahead as planned.
They held talks to end days of protests on Saturday, but the country's main opposition parties say the draft is biased and have boycotted the talks.
Separately, Hisham Qandil, prime minister, told AFP news agency that President Mohamed Morsi was preparing to amend controversial decrees by which he assumed sweeping powers.
Al Jazeera's Sherine Tadros, reporting from Cairo, said the president's dialogue was not going according to plan as "key members of the opposition were notably absent" from the process.
Opposition leaders have been urging Morsi to cancel the December 15 referendum on the draft constitution and rescind the decrees granting him immunity from any oversight.
The National Salvation Front, the main opposition coalition, announced in advance that it would not join Saturday's national dialogue.
The Front's co-ordinator, Mohamed ElBaradei, a Nobel peace laureate, dismissed Morsi's offer as "arm-twisting and imposition of a fait accompli".
For their part, the Muslim Brotherhood and its allies rejected on Saturday the opposition's demands.
The coalition of 13 parties "insist that the referendum on the constitution take place on the scheduled date, with no modification or delay", according to a joint statement read to media by the number two of the Muslim Brotherhood,
Separately, Egypt's military gave warning on "disastrous consequences" if the political crisis gripping the country was not resolved through dialogue.
"The path of dialogue is the best and only way to reach agreement and achieve the interests of the nation and its citizens," the military said in a statement released on Saturday.
"The opposite of that will take us into a dark tunnel with disastrous results."
The military statement was issued as demonstrators fenced off an administrative building in the capital Cairo's Tahrir Square.
It also came after the state-run newspaper Al-Ahram reported that Morsi would soon authorise the armed forces to help police keep order. Cairo and other cities have been rocked by violent protests since November 22, when Morsi issued the contentious decrees.