At least four people have been killed during protests in Cameroon's economic capital of Douala, authorities say, ahead of the official announcement of results in a highly contested presidential election.

The regional governor, Samuel Dieudonné Diboua, said police posts had come under attack and security forces had defended themselves.

Hundreds of supporters of opposition presidential candidate Issa Tchiroma Bakary defied a ban to hold protests in several cities, clashing with security forces.

Tchiroma Bakary has insisted he won the presidential election held on 12 October, challenging incumbent President Paul Biya's 43-year-old hold on power. The ruling CPDM party has dismissed the claims.

Diboua condemned the deadly protests that he described as "premeditated acts of violence" and a serious attack on public order and national security.

"Four people, unfortunately, lost their lives," he said, adding that several members of the security forces were also injured.

"Investigations have been opened so that light can be shed on these unfortunate incidents," he added, while threatening to take action against the instigators.

In Tchiroma Bakary's stronghold of Garoua, a city in the north of the country, police fired tear gas and water cannons at his supporters.

The protesters were denouncing what they said was a plan by the ruling party, the Cameroon People's Democratic Movement (CPDM), to "steal the victory" from the opposition leader.

The demonstrations came after Tchiroma Bakary called on his supporters in the country and the diaspora to march peacefully to "liberate Cameroon".

Authorities have banned gatherings until Monday, when Cameroon's constitutional council is set to announce the results.

In Garoua, the demonstrations began peacefully but quickly turned rowdy when security forces threw teargas on the streets to disperse the hundreds of people that had gathered in support of Tchiroma Bakary.

"We are not here for disorder. We're demanding the truth of the ballot," a placard read.

One protester was seen carrying a banner urging US President Donald Trump to help them.

"We are here to claim our victory. We are making a peaceful march, which is a civil right for all Cameroonians - for everyone," another protester said.

Supporters also took to the streets in the south-western city of Douala. "We want Tchiroma, we want Tchiroma," protesters chanted, Reuters news agency reports.

Tchiroma Bakary previously told the BBC that he would not accept a stolen vote.

He said his team had compiled the overall picture based on results from individual polling stations.

In a video statement posted on social media, Tchiroma Bakary said he had won the election with about 55% of the vote, based on what he said were returns representing 80% of the electorate.

The 76-year-old former government minister broke ranks with Biya, 92, who is seeking another term after 43 years in power.

CPDM has dismissed Tchiroma Bakary's victory claims and several officials have described it as illegal because only the constitutional council can proclaim official results.

Opposition supporters have alleged that the 12 October poll was marred by irregularities, including ballot-stuffing.

Judges on the constitutional council dismissed eight petitions, citing insufficient evidence of irregularities or a lack of jurisdiction to annul results.

Tchiroma Bakary refused to file complaints with the council, whose judges have been appointed by Biya, choosing instead to declare himself the "legal and legitimate president".

Born in Garoua, Tchiroma Bakary trained as an engineer in France before returning to Cameroon to work for the national railway company.

In 1984, he was thrown in jail, accused of being involved in an attempt to depose President Biya. Despite denying the allegation and never being convicted, Tchiroma Bakary spent six years in prison.

He also served as communications minister from 2009 to 2019.

In this role and as the government's spokesperson, he resolutely defended Biya's government during crises such as the Boko Haram insurgency, when the army was accused of killing civilians.

But in June, just four months before the general election, Tchiroma Bakary changed course dramatically, resigning from the government and announcing he would run against Biya for the presidency.