Riot police in Burundi have used water canon, tear gas, and live bullets to disperse crowds protesting against the president's decision to seek a third term.

Witnesses said police clashed with demonstrators in four districts of the capital Bujumbura on Sunday after President Pierre Nkurunziza's government banned protests for or against his move.

At least two people have been killed during the clashes according to the Red Cross, the Reuters news agency reported.

Burundi's ruling CNDD-FDD party nominated Nkurunziza as its presidential candidate on Saturday, prompting hundreds of civil society groups to condemn the move as a "coup" against the constitution, which limits leaders to two terms in office.

Those opposed to a third term also say it goes against the spirit of a peace deal signed in 2000 that has kept Burundi calm for a decade since a civil war between ethnic Hutus and Tutsis in the small nation ended in 2005.

The UN and Rwandan officials say just over 17,000 Burundians have fled into neighbouring Rwanda and the Democratic Republic of Congo since mid-March due to rising fear of violence in the run-up to the June 26 presidential election.

"We deplore the way police acted with violence against a peaceful demonstration," Janvier Bigirimana, a civil society activist, said of Saturday's events.

Interior Minister Edouard Nduwimana said the demonstrations were illegal because the government had banned any protest for or against the president pursuing a third term.

Source: Agencies