The West African nation has been ruled by the military since a September 2021 coup, with junta-appointed Prime Minister Amadou Oury Bah in March saying the military would not honor their commitment to hand over power to civilians by the end of the year.
Oury Bah last week justified the military's continued rule with the need to "rebuild" the country.
The National Alliance for Alternation and Democracy (ANAD), a coalition of parties and associations, urged the junta to respect the timeline for a return to civilian rule by December 31, agreed under pressure from regional bloc ECOWAS.
"ANAD vigorously denounces the junta's renunciation of the commitments to which it freely subscribed before the people of Guinea, ECOWAS and the international community," it said in a statement published Saturday.
"To justify this postponement, all sorts of arguments, each as flimsy as the next, have been put forward. But no one is fooled," it added.
"The postponement of the return to constitutional order will result in the loss of all legitimacy for the junta to lead the transition and will expose our country to major risks," the statement said.
It called on the junta and transitional bodies to organize elections in 2024, urging Guineans to mobilize using "all legal means, including demonstrations in the streets and public spaces to insist this legitimate demand be met."
The junta banned all demonstrations in 2022 and has arrested a number of opposition leaders, civil society members and the press.
"If the junta refuses to accede to this demand, ANAD will demand the establishment of a civil transition to organise the return to constitutional order," the coalition said.