With 33 percent of ballots counted, preliminary results show that the ruling MPLA has grabbed 60.65 percent of the vote, with the main opposition UNITA at 33.85 percent.
Early results from Angola's most tightly contested vote in its democratic history have shown a wide margin for President Joao Lourenco, but the opposition is also claiming a lead.
Ballot counting began after polls closed on Wednesday in the oil-rich nation, where multi-party polls were only introduced in 1992.
Preliminary results published overnight by Angola's electoral commission gave the ruling People's Movement for the Liberation of Angola (MPLA) 60.65 percent of the vote with 33 percent of ballots counted.
The main opposition the National Union for the Total Independence of Angola (UNITA) led by charismatic leader Adalberto Costa Junior was at 33.85 percent.
The MPLA has ruled Angola for nearly 50 years after the country gained independence from Portugal.
Lourenco, a Soviet-educated former general who had promised a new era for the southern African nation when he succeeded veteran leader Jose Eduardo dos Santos five years ago, has trumpeted a list of achievements.
He is credited with making far-reaching reforms, including boosting financial transparency and efficiency in parastatal organisations, fighting sweeping nepotism and corruption, and promoting business-friendly policies to lure foreign investors.
But UNITA's deputy leader Abel Chivukuvuku said the party's own tally showed it was ahead.
"Our poll counting centres ... give us clear provisional indication of a winning trend for UNITA in all provinces of the country," he told a live streamed night conference. "We are confident, calm and tranquil".
The MPLA traditionally wields a grip over the electoral process and state media and opposition and civic groups have raised fears of voter tampering.
Results in past elections have been contested, in a process that can take several weeks.
The election has been overshadowed by Angola's many woes - a struggling economy, inflation, poverty and drought, compounded by the death of a former strongman president.
More than 14 million people were registered to vote.
READ MORE: Oil-rich Angola starts voting in tight race