Cameroon's opposition leader has vowed to resist until the "final victory" over President Paul Biya, calling on his supporters to stay mobilised as a civil society group denounced deaths and arrests in protests in multiple cities.

Biya, the world's oldest state leader at 92, has ruled Cameroon since 1982.

His victory in the October 12 election, announced on Monday, has deepened tensions in the cocoa and oil-producing nation, where critics accuse him of using state institutions to cling to power.

On Wednesday morning, supporters of Issa Tchiroma Bakary, a former government spokesperson turned Biya rival, again took to the streets of the economic capital Douala, which were still strewn with debris and burnt tyres after days of unrest.

At least 23 people have been killed as a result of security forces cracking down on protesters since the weekend, a civil society group known as "Stand up for Cameroon" said in a media briefing on Wednesday.

Reuters could not independently confirm those figures, and a government spokesperson did not respond to a request for comment.

ELECTION RESULTS NOT SUBJECT TO APPEAL

In other parts of the usually bustling city, businesses began to gradually reopen after being closed due to protests and the unrest, while traffic remained sparse.

"The truth of the ballot is clear. We won this election by a large majority. This victory is not mine alone; it belongs to the Cameroonian people," Tchiroma said late on Tuesday in his first public address since the Constitutional Council confirmed Biya's win.

"We remain united, mobilised and will continue to resist until the final victory."

The council's decision is final and not subject to appeal.

Other opposition leaders have alleged widespread fraud, accusations rejected by the government.

MINISTER ACCUSES TCHIROMA OF INCITING VIOLENCE

Interior Minister Paul Atanga Nji accused Tchiroma on Tuesday of inciting the violence and rebellion after prematurely declaring victory on October 13.

"This irresponsible candidate, driven by the desire to push through the plot to disrupt public order, issued repeated calls on social media inciting civil unrest," Nji said in a statement.

He said small groups "often under the influence of drugs" had looted shops and set fire to public buildings. He did not provide any evidence that the protesters had taken drugs.

The Stand up for Cameroon group said arrested protesters and other civilians were being held in "inhumane" conditions.

"The lawyers' testimonies describe swollen faces, bruises, and humiliating treatment inflicted on citizens who sought to exercise their constitutional rights," it said in a statement.

A video circulating on social media showed security forces loading an inert body onto the back of a military truck. In another clip, protesters could be heard chanting, "Give back the corpse."

Reuters could not verify the authenticity of the videos. Security forces directed all questions about accusations over the treatment of detainees to the government.

In Douala, which has experienced some of the worst violence, Nji said many public and private properties had been vandalised.

He said security personnel were injured and attackers killed during clashes.

The governor of the region told a press conference on Tuesday that over 200 people had been arrested in connection with the protests.