It is the first national vote for 16 years in Angola, which is booming because of soaring oil revenues and recovery from civil war. Angola wants the vote to set an example on the continent after flawed polls in Kenya and Zimbabwe.

But Human Rights Watch said in a report that conditions ahead of the September 5 election did not point to a fair ballot.

"The conditions for free and fair elections start long before election day," said Georgette Gagnon, Africa director at New York-based Human Rights Watch, in a report.

"Less than a month before elections, it's clear Angolans aren't able to campaign free from intimidation or pressure. And unless things change now, Angolans won't be able to cast their votes freely."

Angola's government has denied accusations of rights abuses. When asked about the report, Prime Minister Fernando Dias Dos Santos said the polls would take place in a "free, fair and transparent" environment.

The official campaign kicked off on August 5 with 10 parties and four coalitions competing for votes. Each party will be given five minutes of television time each day and 10 minutes on the radio to lure voters.

A total of 5,198 candidates will fight for 220 parliamentary seats in a country that rivals Nigeria as Africa's biggest oil producer, but where two-thirds of the population live on less than $2 a day.

Based on missions to Luanda and four provinces, Human Rights Watch said Angola's government is "failing to fully ensure the right to free elections, and other rights critical to a fair poll, including the freedoms of expression and of assembly".

SECURITY CONCERNS

"Major areas of concern include the government's obligations to safeguard freedom of assembly and expression and access to the media by all stakeholders, and to establish an impartial national electoral body," it said.

"The government has also failed to provide adequate security to political parties participating in elections and to ensure political tolerance and full participation of citizens."

In a rare radio address this month, dos Santos appealed to all political parties to maintain the peace as the country recovers from the 27-year civil war that ended in 2002. He called for an election that is an example to the world.

On Monday, Angolan police detained 13 people campaigning for the main UNITA opposition party in the Rangel area outside the capital Luanda, Catholic Radio Ecclesia reported.

Police told the radio they were accused of holding a rally without notifying authorities and for disrupting traffic.

The prime minister played down the report, calling it an "isolated event".

"Authorities will act when public order is disrupted," he told Reuters on the sidelines of a meeting with South African President Thabo Mbeki in Angola.

Many political observers say UNITA, a former guerrilla movement, is unlikely to unseat dos Santos's ruling MPLA, which holds 129 seats out of 220. UNITA has 70 seats.

The parliamentary poll will be followed by a presidential election in 2009. Dos Santos, who has ruled since 1979, is expected to run.

The European Union has sent 90 election observers to Angola and other missions are expected.