"International resolutions speak of... the formation of a transitional body with full powers, including presidential powers," HNC senior member Asaad al-Zoabi said, adding "Assad should not remain for even one hour after the formation" of this body.
"Bashar and his gang live outside of reality, on another planet... all of Assad's recent declarations are an attempt to evade the question" of the transition, Zoabi told AFP.
Assad made the demand in an interview with Russia's RIA Novosti state news agency published on Wednesday.
The Syrian leader said it would be "logical for there to be independent forces, opposition forces and forces loyal to the government represented" in any transitional body.
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He did not specify which opposition groups should be included in the government but his remarks come with Damascus facing international pressure to compromise at UN-mediated talks aimed at ending the five-year conflict that has killed some 270,000 people.
The talks led by UN envoy Staffan de Mistura paused last week with the sides deadlocked over the fate of Assad, whom the opposition insists must leave power before a transitional government is agreed.
The form of the executive body that would lead Syria until its elections the UN says should be held in 18 months is the main bone of contention between the two sides.
UN Security Council Resolution 2254 vaguely suggests the establishment of a body to head the political transition.
For the regime, this amounts to a government reshuffle in which the opposition is included, but for the opposition it would be a transitional body with presidential powers and no role for Assad.
"We aren't concerned about Assad's interpretation of the transitional period... we must move to a new stage based on a pluralistic regime, civil and democratic," said Zoabi.