Angola's main opposition party, Unita, has asked the country's constitutional court to order a re-run of parliamentary elections.
Unita says voting in the capital, Luanda, has been so chaotic, the poll should be declared illegal.
The ballot was extended into a second day after delays and administrative problems left many people unable to cast their votes.
The elections are Angola's first in 16 years, following the end of a 27-year civil war in 2002.
Fourteen parties are taking part but the contest is mainly between the ruling MPLA party and its long-term rivals, Unita.
Some eight million voters are registered - more than a quarter of whom live in overcrowded conditions in the the capital.
State media reported the polls reopened at 0700 local time (0600 GMT) on Satirday in Luanda. The electoral commission reopened 320 polling stations.
In the lead up to the election, Unita accused the MPLA of intimidating its supporters and dominating state media.
The MPLA has governed Angola since independence from Portugal in 1975. It is widely expected to win.
Thursday was the first day in a week that the state newspaper Jornal de Angola carried no front-page picture of President Jose Eduardo dos Santos.