By Paul Simao

LUANDA, Sept 8 (Reuters) - Angola's ruling MPLA may be declared the winner of the country's disputed parliamentary election as early as Monday when authorities release a round of results expected to show the party with an insurmountable lead.

The MPLA, in power since independence from Portugal in 1975, is winning nearly 82 percent of the vote versus 10.5 percent for UNITA, the largest opposition party in the oil-rich nation.

But UNITA's vow to contest the election due to what it describes as voting irregularities threatens to shatter the fragile political stability that has existed since the end of a 27-year civil war in 2002.

An MPLA victory is likely to be welcomed by investors, particulary oil firms, who are generally comfortable with the government's pro-business policies. Angola rivals Nigeria as sub-Saharan Africa's largest oil producer and its economy grew by 24 percent in 2007.

"It is not going to change the political landscape. The same government will carry on as before and it is very favourable to investors," said Indira Campos, a researcher with London-based think-tank Chatham House.

The international community has been watching the vote closely, hoping that Angola would defy its own history and emerge from the election with political consensus.

That hope dimmed in the chaotic early hours of voting on Friday. Hundreds of polling stations opened late or not at all and many did not have the registration lists necessary to identify voters.

Authorities denied wrongdoing but admitted there were administrative glitches in some areas, particularly in Luanda province, home to 21 percent of the nation's 8.3 million voters.

An extra day of voting was organised on Saturday, but the move failed to satisfy UNITA (National Union for the Total Independence of Angola), which demanded a re-vote and vowed to challenge the vote in the Constitutional Court.

"There are people that were instructed, who were forced to vote, (for) a particular party. I wonder how this was election was free," UNITA leader Isaias Samakuva said in an interview broadcast on South Africa's SAFM radio on Monday.

Samakuva has ruled out any prospect the former rebel group would again take up arms against the government. The long war between the two sides killed half a million people.

OBSERVERS SPLIT

International monitors were split over whether to give the Angolan election a clean bill of health.

Observers from the Southern African Development Community have said the poll was credible, transparent and free. But a European Union mission has raised concerns about irregularities.

The EU team is expected to deliver its report on Monday.

The results, if they hold, represent a stunning collapse in support for the opposition and an overwhelming mandate for the ruling party, which has touted the election as a showcase for the country's recovery from the war.

The MPLA (Popular Movement for the Liberation of Angola) is within reach of a coveted two-thirds of the 220-seat parliament, giving it the power to change the constitution.

In the last parliamentary election in 1992 the MPLA won 54 percent to UNITA's 34 percent.

That poll was overshadowed by a parallel presidential race that ended when UNITA leader Jonas Savimbi withdrew from a second round and resumed the war after accusing President Jose Eduardo dos Santos of cheating.

The UNITA chief was killed in an ambush in 2002. (Additional reporting by Michael Georgy in Johannesburg; editing by xxxx)

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