Cameroon's government called opposition presidential election candidate Maurice Kamto an "outlaw" on Monday after he defied warnings and declared himself victor of weekend polls ahead of official results.
Movement for the Rebirth of Cameroon (MRC) candidate Kamto's dramatic announcement followed Sunday polls marked by violence in restive anglophone regions, low turnout and difficulties staging the ballot in the conflict-torn areas.
"The government is responsible for protecting public order... as soon as someone goes against institutions, they will face the full force of the law," Information Minister Issa Tchiroma Bakary told AFP.
By law each polling station must submit its results, after verification by the Elecam electoral commission, to the Constitutional Court which is responsible for announcing the final, official tally within 15 days.
But at a media briefing in the capital Yaounde, Kamto proclaimed himself president-elect despite being unable to furnish any evidence for his claim.
"I was charged with taking a penalty, I took it, and I scored," he said using a signature football metaphor to applause from supporters.
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"I have received a clear mandate from the people and I intend to defend it until the end." A raft of unofficial results from Cameroon's almost 25,000 polling stations have already begun to circulate on social media.
Opposition candidates had called on their supporters to oversee the tallying process to prevent any fraud that might favour 85-year-old Biya's quest for re-election.
"Times are tough. Rise up and prepare to defend your victory because there are some unbelievable things going on," said outsider opposition hopeful, Cabral Libii, who at 38 was the youngest candidate.
Ahead of the polls, in which 6.5 million voters were eligible to cast ballots, Kamto warned he would "not accept any" result tainted by fraud.
Labour Minister Gregoire Owona, who is the deputy secretary-general of Biya's ruling party, said on Twitter: "I strongly recommend that you don't tie yourself to any violent, insurrection movement".
Tensions were high during the vote and violence was reported in the anglophone regions which have been torn by a separatist insurgency that erupted a year ago.
After voting began Sunday, security forces shot dead three suspected separatists who had allegedly fired at passersby from a motorcycle in Bamenda, the main city in the English-speaking northwest region, a local official said.
In Buea, capital of the anglophone southwest, three fighters of the so-called Ambazonia Republic separatist movement were gunned down on Friday.
Gunfire was heard in the largely deserted town throughout polling day and cars belonging to the state-run Cameroon Tribune newspaper as well as a local official came under fire.
The official suffered non life-threatening injuries, said a medical source.
The violence in the anglophone regions has killed at least 420 civilians, 175 members of the security forces and an unknown number of separatists, according to the International Crisis Group (ICG) think-tank.
"We're bored because no one came to vote, people stayed at home because they're scared," said opposition election observer Georges Fanang in one of Buea's polling stations.
The army also confirmed that voting could not be held in at least one district of the southwest, Lysoka village, because of the insecurity.
"As expected, turnout in the English regions has been particularly low with virtually all the returns we have seen suggesting less than five percent," said ICG analyst Hans de Marie Heungoup.
The poll passed off without incident in the rest of the country.
In the far north region, considered a "key" to the election because of its large population, very few opposition election observers deployed to remote polling stations, witnesses said, raising fears of possible fraud.
The region has been rocked by violent attacks carried out by Nigeria-based Boko Haram jihadists despite US efforts to equip and train Cameroonian forces.