No presidential candidates in Guinea-Bissau have secured enough votes to avoid a run-off after Sunday's poll, the preliminary results show.
Ex-Prime Minister Carlos Gomes narrowly missed winning an overall majority in the first vote, scoring 49%, according to the election commission.
In the next round he should face ex-President Kumba Yala, who secured 23%.
On Tuesday, five candidates, including Mr Yala, rejected the vote claiming the poll had been fraudulent.
The candidates said they plan to submit a formal complaint, reports the BBC's John James in the capital, Bissau.
Guinea-Bissau has been plagued by numerous coups since gaining independence from Portugal in 1974.
The country has also been destabilised by the booming illegal drugs trade from Latin America which uses it as a staging post for the European market.
Sunday's election was held after the death of the last President, Malam Bacai Sanha, following a long illness.
The head of the UK observer mission, Peter Thompson, said his team was content with the way in which the first round had been conducted.
The second round is set to be held next month, although the date has not been confirmed.
It remains unclear whether Mr Yala will agree to participate in the run-off.
National Assembly speaker Manuel Serifo Nhamadjo scored 16% in the first round and Henrique Rosa achieved 5%.
Hours after Sunday's polls closed, the former head of military intelligence, Col Samba Djalo, was shot dead by uniformed men.
It is not clear why he was shot but he was previously linked to a 2009 bombing which killed the then army chief.