BUJUMBURA - New political violence in which at least one person died heightened tensions in Burundi ahead of a presidential election on Monday in which only the current head of state is standing.

The opposition is boycotting the election despite calls by regional organisations to end their protest against President Pierre Nkurunziza.

"Five unidentified gunmen attacked a family... in Ruyigi, east of the capital Bujumbura, killing a man and seriously wounding his wife," national police spokesman Pierre Chanel Ntarabaganyi told AFP.

It was not immediately clear if the attack was politically motivated.

Ntarabaganyi said a grenade was hurled at the home of a local ruling party official in Bujumbura overnight, wounding his son.

Another grenade attack targeted a vehicle blaring electoral propaganda for the ruling party in the central town of Gitega, causing material damage.

A high-stakes electoral marathon, which had been seen as a test of the central African nation's democratic credentials, started with May 24 local council polls which were comfortably won by the ruling party.

The voting process got the nod from foreign observers but was rejected as fraudulent by all opposition parties, which subsequently announced they would boycott the remaining elections.

According to the police, at least six people have been killed since the start of the dispute and more than 60 wounded.

Defence Minister Germain Niyoyankana this week urged the country's politicians to preserve the 2006 ceasefire that ended Burundi's civil conflict.

"Not a night goes by without people being killed... My concern is that my country descends back into the violence that prevailed between 1993 and 2003," he said, referring to the height of the civil war.

"The risk is real," he added.

Burundi is packed with recently demobbed soldiers and rebels, while grenades are available on the black market for one dollar.

President Nkurunziza, 45, is guaranteed a win in Monday's presidential poll as he is the only candidate standing after six opposition competitors pulled out.

Jean Ping, the head of the African Union's executive arm, issued a statement Friday calling for calm.

"He is appealing urgently to all the political actors to refrain from any action likely to lead Burundi back to the horrors of the past and that could jeopardise the political and democratic progress of the past few years," the statement said.

Ping called on Burundi's opposition parties to "use only legal means to resolve disputes linked to the local elections."

The East African Community (EAC) -- of which Burundi is a member, together with Rwanda, Kenya, Uganda and Tanzania -- also urged the opposition to rejoin the electoral process.

"There is no perfect democracy, there may have been irregularities, but as far as the East African Community is concerned, the message is clear: the process should continue," Rwandan Foreign Minister Louise Mushikiwabo told reporters.

She spoke in the name of the regional body following a meeting Friday with Nkurunziza.