They demanded an independent probe because of alleged widespread irregularities and have threatened to boycott the Constituent Assembly. Party spokesman Agni Sapkota said the party will decide its next move during a meeting on Monday.
Independent observers, including former U.S. President Jimmy Carter, have said the election was free and fair of any irregularities.
The Maoists fought government troops between 1996 and 2006, but then gave up their armed revolt, joined a peace process and mainstream politics. Their fighters joined the national army.
Political leaders in the last assembly elected in 2008 disagreed over who got to lead the nation and on creating a federal system divided by ethnic groups or by geography. The resulting power vacuum has left Nepal without a proper constitution.