Angolans are due to vote for a new president and parliament, in the second national elections since the country's 27-year civil war ended a decade ago.
President Jose Eduardo dos Santos is expected to win another five-year term.
The polls are the first held under a new constitution that abolished direct presidential elections. Instead, the head of the winning parliamentary list becomes president.
Opposition parties have called for a delay, alleging irregularities.
The main opposition party, Unita, has already expressed concern about a lack of transparency, especially the failure to publish a full electoral roll.
The National Electoral Commission (NEC) denied there are problems.
But hours before polls opened, it emerged that thousands of Angolans are unsure about where to vote, and many more could not find their name on voting lists, says reporter Louise Redvers in Luanda.
Unita spokesman Alcides Sakala has said the confusion vindicated his party's concerns, and called for the vote to be delayed.
"The NEC has done nothing to correct these problems," he said
A leading oil producer, Angola has witnessed an economic boom since emerging from the 27-year civil war that ravaged the country after independence from Portugal in 1975.
Analysts predict an easy win for Mr Dos Santos' governing Popular Movement for the Liberation of Angola (MPLA), which won more than 80% of the vote in the last legislative election four years ago.
The MPLA campaign stressed continuity and is being run under the slogan "Grow more, distribute better" - a response to criticism that the country's oil wealth remains concentrated in the hands of a small elite.
In its campaign, Unita has been trying to benefit from growing discontentment with the MPLA, as well as the fact that despite strong growth, half the population remains in poverty.
However, the party faces stiff competition from a new party, Casa, formed in March by former Unita politician Abel Chivukuvuku, who is targeting the youth vote and promoting an anti-corruption agenda.
On Thursday, Unita leader Isaias Samakuva unsuccessfully tried to meet Mr dos Santos to discuss his party's complaints.
Angelo Kapwacha, from the Civil Society Electoral Process Reflection, a group of non-politically aligned national observers, said the country was not ready for the vote.
"Some people do not know where they are supposed to be voting, others have been told to go to polling stations very far from their homes and there is a big confusion."
MPLA supporters have dismissed the claims of attempted fraud, saying their party does not need to cheat, and that the allegations are merely a disguise for a lack of policies.