The opposition said on Sunday it was gathering evidence of fraud.
Many of the ballots yet to be counted were in the capital Luanda, home to nearly a third of the country's registered voters, where opposition parties were performing better. Angolans voted Friday for 220 members of Parliament, with the leader of the winning party awarded a five-year term as president.
That left Dos Santos all but certain to extend his nearly 33-year rule over Africa's second-largest oil producer, despite a series of urban protests demanding that more of the nation's wealth go toward helping the 55% of the population living in abject poverty.
Those frustrations appeared to have taken a toll on his party in Luanda, where Dos Santos's ruling People's Movement for the Liberation of Angola (MPLA) had 57% of the vote with about one-third of polling stations reporting. Nationwide, the MPLA had over 74% of the vote.
The main opposition, the Union for the Total Independence of Angola (Unita), had nearly 18% of the national vote, and 27% in Luanda. The breakaway Casa party, formed by dissenters from both the main parties, had around 4.6% of the national vote but more than 12% in the capital.
Unita leader Isaias Samakuva has long criticised the electoral process, saying the voters roll hadn't been authenticated while some 2 000 of his party's observers were denied accreditation to polling stations. A statement on the party's website said both Unita and Casa "are preparing to present documents that prove the results presented by the National Electoral Commission are not the same as those tabulated inside the polling stations in different parts of the country".
The vote count began immediately after polls closed Friday, in the presence of election officials and monitors from the different parties.
Few international observers watched over the vote. Teams from the African Union and the Southern African Development Community planned to present their initial findings later Sunday. – AFP