MAPUTO, Oct 22 (Reuters) - Mozambique's President Filipe Nyusi is heading for a landslide victory in elections, according to a preliminary estimate by an observer group on Monday, in a contest the opposition claims has been tarnished by "mega fraud".
Based on data from almost 2,500 polling stations out of a total of more than 20,000, the the Electoral Institute for Sustainable Democracy in Africa (EISA) estimates Nyusi will win 70.9% of the vote.
Though official results have yet to be announced, former guerrilla movement turned main opposition party Renamo has already rejected the outcome, decrying irregularities in the process from voter registration through to counting.
Perceived fraud and an unhappy Renamo threaten a fragile peace accord signed just months ago between its presidential candidate, Ossufo Momade, and Nyusi.
EISA programme officer Domingos Rosario said that during its count, the institute found that in many polling stations across seven different provinces, the number of votes greatly exceeded the number of voters registered at that station.
The official count for the Oct. 15 presidential, provincial and legislative poll, which will decide who controls a nation on the cusp of becoming a top global gas exporter, is still ongoing. A spokesman for the Electoral Administration Technical Secretariat did not respond to multiple requests for comment.
Two-thirds of polling stations have been processed, representing just over 36% of the more than 13 million potential ballots from registered voters, according to data on the electoral commission's website, which has barely changed since Sunday.
So far, Nyusi has taken 74.6% of the presidential vote versus 20.2% for his main rival, Renamo candidate Ossufo Momade, the website shows.
EISA estimates Momade will end up with around 21.4% of the vote while Daviz Simango, of smaller rival the Mozambique Democratic Movement, will secure 6.9%, Rosario told a news conference.
Renamo and Nyusi's party, the ruling Frelimo, fought on opposite sides of a 16-year civil war that ended in a truce in 1992, but the former foes have clashed sporadically since.
The pact was meant to bring a formal and definitive end to four decades of violence between the two, shoring up Mozambique's stability just before blockbuster gas projects in the gas-rich north led by the likes of Exxon Mobil Corp and Total take off.
Instead, it is at risk of collapse as tensions rise. Nyusi and Frelimo, which has controlled Mozambique since independence from Portugal in 1975, were widely expected to win the election but not by such a significant margin.
Frelimo spokesman Caifadine Manasse said Renamo regularly complains about elections, despite being part of all the electoral bodies and having delegates at voting stations.
"Renamo is not serious about democracy," he said.
Reporting by Manuel Mucari in Maputo and Emma Rumney in Johannesburg; Writing by Emma Rumney; Editing by Giles Elgood