Niger is heading for the second round of a presidential vote on February 21.
Former president of Niger Mahamane Ousmane, who was driven from power by a military coup after three years, is locked in a second-round election runoff with favoured ruling party candidate Mohamed Bazoum.
Opposition parties have now urged candidates to rally behind Ousmane, the runner-up to Mohamed Bazoum in last month's first round.
Gathered in a coalition called Cap 20-21, 18 parties launched "an appeal for civic responsibility from all the political forces in Niger... to join the final combat."
The manifesto was first read out on Monday to more than 3,000 people in Niamey's main sports arena.
Mohamed Bazoum, a former interior minister supported by outgoing President Mahamadou Issoufou, took a strong lead in the first round of voting on December 27, winning 39.33 percent of ballots cast.
Ousmane won 16.99 percent in a crowded field of 30 candidates.
Both frontrunners will need to haggle with other candidates to secure their support, a process likely to be complicated in the poor Sahel country where alliances are fickle.
Issoufou is voluntarily standing down after two five-year terms in office.
Handing over to another democratically-elected newcomer will be a first in Niger after a history studded with coups d'etat since independence from France in 1960.