In a statement read by its spokesman, the National Salvation Front also called for mass protests on Tuesday against a draft constitution that it said lacked consensus and did not properly represent the interests of women and minorities.
"The National Salvation Front rejects the referendum to be held on December 15," Hussein Abdel Ghani told a news conference. "We are against this process from start to finish."
A statement from the Front said: "Holding a referendum now in the absence of security reflects haste and an absence of a sense of responsibility on the part of the regime, which risks pushing the country towards violent confrontation."
The main leaders of the Front, its coordinator Mohamed ElBaradei, former Foreign Minister Amr Moussa and leftist Hamdeen Sabahy, did not attend the news conference.