Cameroon opposition candidate Issa Tchiroma declared victory late on Monday in the country's October 12 presidential election, urging President Paul Biya to accept defeat and "honour the truth of the ballot box".

"Our victory is clear. It must be respected," Tchiroma said in a speech on his Facebook page from his hometown of Garoua in the north of the Central African state.

"The people have chosen. And this choice must be respected."

Tchiroma, a former government spokesperson and employment minister in his late seventies, broke ranks with Biya earlier this year and mounted a campaign that drew large crowds and endorsements from a coalition of opposition parties and civic groups.

Without naming Tchiroma, Biya's Cameroon People's Democratic Movement (CPDM) on Tuesday condemned the self-declared victory as a "grotesque hoax," adding that only the Constitutional Council is enabled to proclaim results.

It is "an inadmissible act in a state governed by the rule of law," CPDM spokesperson Jacques Fame Ndongo said in a statement.

The Minister of Territorial Administration also criticized Tchiroma in a separate statement and promised to maintain public order and ensure the protection of people and their property during the electoral period.

Biya, 92, the world's oldest serving head of state, is seeking an eighth term after 43 years in power. Analysts had expected his control over state institutions and a fragmented opposition to give him an edge in the election, despite growing public discontent over economic stagnation and insecurity.

Tchiroma praised voters for defying intimidation and staying at polling stations late into the night to protect their ballots.

"I also thank candidates who have already sent me their congratulations and recognised the will of the people," Tchiroma said.

"We have placed the regime before its responsibilities: either it shows greatness by accepting the truth of the ballot box, or it chooses to plunge the country into turmoil that will leave an indelible scar in the heart of our nation," he warned.

Cameroon's electoral law allows results to be published and posted at polling stations, but final tallies must be validated by the Constitutional Council, which has until October 26 to announce the outcome.

Tchiroma said he would soon release a region-by-region breakdown of vote tallies compiled from publicly posted results.

"This victory is not that of one man, nor of one party. It is the victory of a people," he said.

He also called on the military, security forces and government administrators to remain loyal to the "republic, not the regime".

Minister of Territorial Administration Paul Atanga Nji warned over the weekend that any unilateral publication of results would be considered "high treason."

Cameroon's single-round electoral system awards the presidency to the candidate with the most votes. More than 8 million people were registered to vote in the election.