It said it took the decision because of "the fundamental role... Given to personalities linked to the regime" at the conference in Riyadh.
It named the Syria-based National Coordination Committee for Democratic Change, which is generally tolerated by the regime and participated in talks organised by Moscow on the conflict in 2014 and 2015.
Allied with the Qaeda-affiliated Al-Nusra Front, Ahrar al-Sham said it "rejects the outcomes" of the meeting which "did not affirm the identity of our Muslim people" in Syria.
Ahrar al-Sham had agreed to attend the Riyadh talks despite the "lack of representation of jihadist factions at a level matching their... Role" on the ground in Syria.
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But the Islamist group had warned it "will not accept the results of this conference" unless they include "cleansing Syrian territories of the Russian-Iranian occupation and sectarian militia supporting them."
Russia has been conducting air strikes in support of Assad for more than two months, while Iran has provided military assistance.
Ahrar al-Sham had insisted the priority should be overthrowing the regime, preserving Syria's unity and rejecting sectarianism.
A source in the National Coalition, the main opposition group which is based in Istanbul, said today that the delegates had agreed on a set of basic principles, including ensuring Syria is a "pluralist and civil state" and guaranteeing the country's territorial integrity.
The source, whose information was confirmed by a second opposition figure, said delegates also agreed on "the preservation of state structures, the restructuring of military and security bodies, and the rejection of terrorism and the presence of foreign combatants".