Angola's first election for 16 years has been condemned by opposition parties who say the vote is chaotic.
The poll, largely a race between the ruling MPLA and opposition UNITA, has been keenly watched by the international community because of controversy marring recent African polls and Angola's emergence as one of the world's major oil producers.
It was unclear if the extension of Angola's voting into a second day on Saturday would satisfy UNITA, which earlier described the poll as a "mess" and said it should be re-held.
Scores of polling stations had failed to open early on Friday and others did so hours late. Problems with voter registration lists were cited as the main cause of the delays.
"The system practically collapsed and we have to do something to recover the process," UNITA leader Isaias Samakuva said.
Two smaller parties backed Mr Samakuva's call for voting to be reheld.
Caetano de Sousa, the president of the National Electoral Commission, said voting would continue on Saturday in the province of Luanda, where many of the problems occurred.
The province is home to 21% of the estimated 8.3 million registered voters and is considered a stronghold for the MPLA, which is widely expected to win the election and extend its uninterrupted 33-year rule.
Angolans last voted in 1992 in joint parliamentary and presidential elections. The presidential race was aborted after UNITA leader Jonas Savimbi withdrew, accusing MPLA leader Jose Eduardo dos Santos of cheating.
Savimbi then led his supporters back into the bush where the rebel group resumed its war against the government. An estimated half a million people died in the conflict, which ended after Savimbi was killed in an ambush in 2002.