Three people were killed in clashes with the police yesterday and three others died of their injuries overnight, Alexis Manirakiza told The Associated Press by phone from Bujumbura, Burundi's capital.
Seven more people had been wounded, he said. Bujumbura has been hit by protests since Sunday after the ruling party nominated President Pierre Nkurunziza for another term, which many say is unconstitutional.
Presidential elections are scheduled for June 26 and political tensions have been rising since the start of the year.
"I will stop coming to the road when the president says he has abandoned (the third term bid,)" said demonstrator Flaurice Nkurunzinza.
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More than 250,000 people died in Burundi's civil war between Hutu rebels and a Tutsi-dominated army. The war ended in 2003.
Those who oppose Nkurunziza, an ethnic Hutu, running for a third term include members of his own party, lawmakers, the clergy, student groups and civil society.
Another 4,000 Burundians are sheltering in Congo, according to the UN refugee agency.
Human Rights Watch has accused the Imbonerakure of committing serious rights abuses.
Amid the street demonstrations, there is "widespread intimidation and efforts to shut down media outlets, as well as intimidation and arrests of rights activists," the UN Office for the Coordination of Humanitarian Affairs reported today.
In six months 350,000 people in Burundi could be in need of humanitarian assistance, according to the UN group, which gave the figure as a worst case scenario.