Angola's main opposition party on Thursday filed a case with the constitutional court to seek the annulment of last week's election in which the ruling MPLA was declared the winner, Portuguese news agency Lusa said.

After the most closely fought election since independence from Portugal in 1975, the electoral commission declared the People's Movement for the Liberation of Angola (MPLA) the winner, prolonging its nearly five decades of uninterrupted rule and handing President Joao Lourenco a second term.

Just over 51% of voters had supported the ex-Marxist MPLA, it said. The National Union for the Total Independence of Angola (UNITA), its longtime opponent and former civil war enemy, took about 44%, its best result ever, according to the commission.

UNITA has repeatedly said that it does not recognise the results of the vote, and that various complaints have been filed with the electoral commission. The party has cited discrepancies between the commission's count and the party's own tally.

The vice-president of UNITA's parliamentary group told Portuguese broadcaster RTP that in Luanda, Angola's most populous province, the party was 187,000 votes short.

The commission has denied any wrongdoing, and insists the election was fair and transparent.

UNITA decided to take the matter to the court after its complaints were dismissed by the commission on Tuesday.

The party, led by Adalberto Costa Junior, has yet to comment on the court case but a source familiar with the matter confirmed to Reuters that it was filed on Thursday afternoon.

The court has 72 hours to inform all interested parties, and another 72 hours to rule on the issue. Analysts say the ruling party controls the court, which is led by a former MPLA member, but the government says it exercises its powers independently.

Portugal's Lusa news agency cited a source close to UNITA's leadership who said the case listed "several complaints, which amount to illegalities" committed by the commission.

Lourenco, 68, has pledged to press on with reforms in his second term, including privatising poorly run state assets and continuing to clean up corruption.

But so far his reforms have failed to create a fairer distribution of Angola's vast oil wealth - it is Africa's second largest producer - which remains mostly in the hands of a few well-connected MPLA officials.