At least six people, including one policeman, were killed in Burundi's capital on Wednesday, witnesses and a police spokesman said, in the latest violent clash between police and residents since Monday's parliamentary election.

Burundi has been locked in its worst political crisis since its civil war ended a decade ago, with protests erupting in late April against President Pierre Nkurunziza's bid to seek a third term in office. Dozens have been killed.

Monday's parliamentary election was boycotted by the opposition, and the presidential vote, scheduled for July 15, also comes in the face of widespread protests.

The incident occurred on Wednesday morning in the Cibitoke neighborhood, in the outskirts of the capital Bujumbura, when police were conducting patrols, said police spokesman Pierre Nkurikiye. He said that four people had been arrested and "many, many" guns and grenades had been seized.

"Six people have between killed. Five were criminals who attacked the policemen while they were on their patrol," he said.

Witnesses at the scene said the victims included a man and his two sons, aged 20 and 22.

"This is a war that has started. We have no peace at all," said a witness who did not identify himself but said he had lost a friend in the incident.

Throughout the day on Wednesday, sporadic gunfire could be heard in several flashpoint districts in Bujumbura, marring celebrations marking the independence dayholiday in the former Belgian colony in east Africa.

In a separate incident, at least two policemen were injured during a grenade attack, witnesses said.

Nkurunziza's decision to seek a third term, and the Burundian election commission's (CENI) insistence on moving ahead with planned elections, have been met with broad condemnation.

The United States and other Western powers have criticized Monday's poll, saying the conditions were not right to stage a fair election.

CENI has said it expects results from the poll to be announced on Wednesday or Thursday.

Although the opposition coalition did not campaign and boycotted the race, names of the parties were still on the ballot paper. A CENI official told state radio their votes would be counted and they would be awarded any seats they won.

The government has pressed on with the election schedule despite the turmoil. Opponents say the president's attempt to stand again violates the constitution.

About 140,000 people, more than 1 percent of the population of 10 million, have fled across the country's borders, stoking concern in a region with a history of ethnic conflict, particularly in Rwanda, where 800,000 people were killed in 1994.