President Jose Eduardo dos Santos, 70, is expected to win another five-year term - he has been in power since 1979.

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.

Hours before polls opened, it emerged that thousands of Angolans were unsure about where to vote, and many more could not find their name on voting lists.

The National Electoral Commission (NEC) denied there were problems.

Unita has expressed concern about a lack of transparency, especially the failure to publish a full electoral roll.

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 2002, after the end of the civil war that had ravaged the country after independence from Portugal in 1975.

It is now Africa's third largest economy.

Analysts predict an easy victory for Mr dos Santos' 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 and lack access to basic services such as water and electricity.

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 opposition allegations are merely a disguise for a lack of policies.