Bujumbura - Police in Burundi on Friday fired tear gas and water cannon to disperse protesters calling for the president not to run for a third term, as tensions rise in the central African state.
Opposition parties are concerned at incumbent President Pierre Nkurunziza's expected bid to seek a third term in office, despite the constitution stating that a president can only be elected twice.
Around a thousand opposition activists attempted to gather in the centre of the capital Bujumbura, but police blocked their way, briefly arresting several.
Some protesters threw stones and police responded with tear gas and water cannon, before calm was restored to the city.
At least two policemen were injured.
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Chauvineau Mugwengezo, president of the UPD opposition party and one of the organizers of the protest, criticised the police for stopping the marches.
"We will continue until Pierre Nkurunziza gives up plans to violate the constitution," he said.
But deputy police chief Godefroid Bizimana said the protesters had not got permission to demonstrate, and that police had been "doing their job by preventing them from disturbing public order."
Burundi, a small landlocked nation in Africa's Great Lakes region, emerged in 2006 from a brutal 13-year civil war.
Next month it will hold a general election for lawmakers before a presidential poll in June.
UN rights chief Zeid Ra'ad Al Hussein on Wednesday warned that the country is at a "crossroads" between a fair vote that would boost the country and a route back to its "horrendously violent past".
More than 8 000 Burundians have fled in the past two weeks to neighbouring Rwanda and Democratic Republic of Congo, the UN refugee agency said Friday.
UNHCR expressed concern that the numbers of refugees could swell "with more political tension rising and more acts of violence being reported."
Many are fleeing threats by the pro-government militia Imbonerakure, the youth wing of the ruling CNDD-FDD party.
Zeid said that the "dangerous" Imbonerakure "appears to be operating increasingly aggressively and with total impunity," as he called for the government clamp down on its activities.