At least six people have been killed in street clashes between the police and civilians demonstrating against the Burundian president's bid for re-election for a third term, a spokesman for the Burundi Red Cross has said, as hundreds continued to gather in the streets of the capital despite a heavy military presence.

Three people were killed in clashes with the police on Sunday and three others died of their injuries overnight, spokesman Alexis Manirakiza told The Associated Press news agency from Bujumbura, Burundi's capital.

Seven more people had been wounded, he added.

Bujumbura has been hit by protests since Sunday after the ruling party nominated President Pierre Nkurunziza for another term, which many say is unconstitutional.

Hundreds of demonstrators erected barricades and set tyres alight in Bujumbura on Monday. The military was deployed on the streets after protests began on Sunday.

In what appears to be the latest development of the police crackdown on dissent, the country's leading human rights activist Pierre-Claver Mbonimpa has been arrested on Monday, his lawyer said.

Armel Niyongere, said Mbonimpa had not been informed of the charges against him, but said he believed "the arrest is linked to his call for demonstrations today".

'Brutally arrested'

A witness, who asked not to be named, said Mbonimpa was arrested "brutally" during a police raid on the headquarters of a media association.

An arrest warrant has also been issued for Vital Nshimirimana, head of a prominent NGO forum and leader of the campaign to block a third presidential term, sources told the AFP news agency.

Presidential elections are scheduled for June 26 and political tensions have been rising since the start of the year.

Burundi's constitution says the president "is elected by universal direct suffrage for a mandate of five years renewable one time", but Nkurunziza's supporters say he is eligible to serve a third term because he was first installed as president in 2005 by parliament to lead a transitional government, and not by a popular vote.

Those who oppose Nkurunziza running for a third term include members of his own party, lawmakers, the clergy, student groups and civil society.

More than 10,000 Burundians have fled to neighbouring Rwanda, citing pressure to support Nkurunziza's party.

Others alleged violence by the ruling party's youth wing, known as Imbonerakure, according to the UN refugee agency.

Human Rights Watch has also accused the Imbonerakure of committing serious rights abuses.

Source: Agencies