Vote counting is still under way under increased security after former military intelligence chief Colonel Samba Diallo was shot dead just hours after polling closed on Sunday in the volatile west African nation.
As the United Nations appealed for calm, five leading presidential hopefuls said the vote was riddled with fraud and irregularities, despite observers having given it a clean bill of health.
“For us, the election was not transparent, fair or credible. We have come to the conclusion that fraud was committed,” said Augusto Gomes, a spokesman for the candidates.
Former president Kumba Yala, who is seeking the top job again after being ousted in 2003 following three years at the head of a regime plagued by instability, demanded the election be cancelled.
“We demand the cancellation of the election because there were irregularities in the process,” said Yala, who is currently placed second in initial vote results.
“Whoever publishes the results will be held responsible for what happens in this country,” he added.
Provisional election results are expected this weekend. If no candidate wins outright, a second round will be held between April 22-29
The other presidential hopefuls making the accusations include third-placed lawmaker Manuel Serifo Nhamadjo and another former leader Henrique Rosa, who is coming in fourth.
Initial poll results put ex-prime minister Carlos Gomes Junior, 62, of the ruling African Party for the Independence of Guinea and Cape Verde (APIGCV) in the lead.
In a joint statement, the five opposition candidates said their accusations centred on the use of sham voting registers and fictitious polling stations, double voting, fake voters cards and the movement of voters from one area to another.
They said these “abnormal events” were perpetrated by the government and National Elections Commission.
Sunday’s election has been seen as a key test of Guinea-Bissau’s commitment to stability after decades of turmoil marked by coups and deadly score-settling between the army and state.
UN Secretary General Ban Ki-moon has appealed to the candidates and their supporters “to refrain from any action that could hamper the electoral process, and to respect the official results”.
The country’s international partners such as the United States and European Union are also looking for signs of stability to reinstate vital support to security sector reform which was frozen after an army mutiny in 2010.
Guinea-Bissau achieved independence from Portugal in 1974, the only west African nation to do so through armed combat.