Conditions do not exist in Angola for a free and fair vote in polls on Friday, the main opposition party has said.
Unita said a failure to publish a full electoral roll was one of its greatest concerns about a lack of transparency.
A local pressure group has called for a delay in the poll, but the electoral commission denies there are problems.
Under the newly adopted constitution, a parliamentary majority for President Jose Eduardo dos Santos' party would give him a new five-year term.
Analysts say the governing ruling Popular Movement for the Liberation of Angola (MPLA) is expected to win an easy victory.
These are the second elections to be held in Angola - one of Africa's major oil producers - since the end of a 27-year civil war that ravaged the country after independence from Portugal in 1975.
Journalist Louise Redvers in the capital, Luanda, told the BBC the month-long election campaign has been lively, colourful and mostly peaceful, with the exception of a few isolated incidents of violence in rural areas, but the vote itself may not run so smoothly.
The failure to publish a full electoral list - despite a legal requirement - has created confusion about where people should vote and the lack of transparency has prompted allegations that the vote is being fixed in the government's favour, she says.
Unita's leader, Isaias Samakuva, has said he wants to meet Mr dos Santos to discuss the various problems, the AFP news agency reports.
"We are very concerned," Unita spokesman Alcides Sakala told the BBC.
"This election is not being organised in an open way according to the law and the correct processes are not being followed. This makes us worry that fraud could be committed."
Angelo Kapwacha from the Civil Society Electoral Process Reflection, a group of non-politically aligned national observers, said the oil-rich nation is 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."
Along with the National Electoral Commission, MPLA supporters have also dismissed the claims of attempted fraud, saying their party does not need to cheat and the allegations are merely to disguise a lack of policies.
The MPLA is campaigning on a ticket of continuity under a slogan of "Grow more, distribute better" - a direct response to criticism that the country's vast oil wealth has stayed in the hands of a small elite.
"These allegations are totally groundless. We organised free and fair elections in 2008 so now we will show the world we can do it again," party activist Vieira Lopes, 45, told the BBC.
On the sidelines of a rally in Luanda on Wednesday, 45-year-old pilot Jose Cassule told the BBC: "I am not worried about fraud. The voting assemblies have people from all of the parties there to observe and we have processes in place to ensure things run properly.
"The only reason the opposition are saying this is because they can't win the election, that is all."
During its campaign Unita has been trying to capitalise on a growing discontentment with the MPLA and the fact that despite the country's strong oil-driven economic performance, half the population remains in poverty, Louise Redvers reports.
However, the party faces stiff competition from a new party, Casa, formed in March by charismatic former Unita politician Abel Chivukuvuku, who is also vying for the youth vote and promoting an anti-corruption agenda, she says.