Guinea Bissau's former prime minister Carlos Gomes Junior scored 49 percent in a presidential election, poll authorities said on Wednesday, well ahead of his main rival but just short of the absolute majority needed to avoid a second round.
But runner-up Kumba Yala, an ex-president who scored 23 percent according to preliminary results read out by the election commission, immediately cast doubt over the process by demanding the poll be annulled on grounds of fraud.
"He is maintaining his position," a spokesman for Yala said of an opposition complaint that Sunday's election in the restive West African state was marred by fraud and should be scrapped.
Guinea Bissau has a long history of coups and assassinations since independence from Portugal in 1974, and its weak policing and impenetrable coastline of mangrove-lined islands has made it a haven for drug smugglers.
The election, held to find a successor to president Malam Bacai Sanha who died of a long illness in January, was aimed at drawing a line under that. While voting was peaceful, it was clouded hours later by the killing of former military spy chief and by the subsequent vote fraud allegations.
The ramshackle capital Bissau has been abuzz with conspiracy theories since Samba Diallo was shot dead by unknown gunmen near his residence hours after polling stations closed on Sunday.
Diallo was an ally of Gomes Junior, but was also widely feared during his time as intelligence chief for his alleged role in previous coups and assassinations.
Gomes Junior has had a rocky relationship with the military since April 2010, when a military mutiny ousted his ally Jose Zamora Induta from the post of Army Chief of Staff, and replaced him with Antonio Indjai.
Yala shares the Balanta ethnicity with a quarter of the population and most of a military, which is believed by many to be complicit in the drugs trade.
The leading candidates have promised to make fighting drugs a priority, but observers say doing so will require more foreign aid.