Mozambique's incumbent President Filipe Nyusi has won a landslide victory in an election it was hoped would calm tensions in a nation soon to become a top global gas exporter, but has instead stoked divisions as opposition parties cry foul.

Nyusi secured 73% of the vote in the presidential race, the National Election Commission (CNE) said on Sunday, while his party, the ruling Frelimo, also won big in the legislative and provincial contests.

His main rival Ossufo Momade, of former guerrilla movement turned main opposition party Renamo, trailed behind with 21.88% of the vote, CNE Chairperson Abdul Carimo told a news conference.

During his second five-year term, Nyusi will be responsible for overseeing a gas boom led by oil giants such as Exxon Mobil Corp and Total, battling a festering Islamist insurgency and delivering on a peace deal signed two months ago.

It was hoped the October 15 poll could set the seal on the fragile pact, designed to put a definitive end to four decades of violence between Frelimo and Renamo. The two fought a 16-year civil war that ended in a truce in 1992 but have clashed sporadically since.

Instead the deal is at risk of falling apart as opposition parties reject the results, claiming they were tarnished by fraud, violence and irregularities from the outset. Frelimo says the elections were free and fair.

Analysts say a return to all-out conflict is highly unlikely even if the deal collapses, but low-level violence, including from an armed group of breakaway former Renamo fighters, could worsen. That could suck the government's focus and resources away from the insurgency in the gas-rich north.

"SHAMEFUL"

Eight members of the CNE, which is made up of Frelimo and opposition party representatives, voted against accepting the results earlier this week, verses nine in favour.

Speaking after the conference, Fernando Mazanga, a Renamo member of the CNE, said they distanced themselves from the results because of irregularities.

"It is shameful and a disgrace what we are witnessing here," he said.

Daviz Simango, of the third largest party the Mozambique Democratic Movement (MDM), secured just over 4% of the presidential vote

Earlier on Sunday, he said MDM saw the election outcome as "null and void". Renamo has also already rejected the results, and a number of observer groups have raised concerns.

Frelimo won 184 out of 250 seats in parliament, verses 60 for Renamo and 6 for MDM, Carimo said.

Frelimo also won a majority in all 10 of Mozambique's provinces in the provincial poll - a contest seen as central to the survival of the peace pact.

For the first time, provincial governors will be appointed by the majority party in each province rather than the government - a key demand of Renamo during peace talks.

The party had wanted to win control of a number of provinces in Mozambique's centre and north to achieve this long-thwarted ambition for influence, but has instead come away empty handed.