Police in Burundi have fired teargas and water cannon to disperse protesters calling for the president not to run for a third term, as tensions continue to rise in the central African country.

Opposition parties are concerned over the incumbent president, Pierre Nkurunziza's expected bid for a third term in the June elections, despite the country's constitution stating a president can only be elected twice.

Around a thousand opposition activists attempted to gather in the centre of the capital Bujumbura on Friday, but police blocked their path, briefly detaining several protesters.

Some protesters threw stones, prompting police to respond with tear gas and water cannon, eventually restoring calm.

At least two policemen were injured in the clashes.

Chauvineau Mugwengezo, president of the opposition UPD party, criticised the security clampdown and called for more demonstrations.

"We will continue until Pierre Nkurunziza gives up plans to violate the constitution," he said.

Deputy police chief Godefroid Bizimana said the rally had not received prior authorisation and that the police were just "doing their job".

Burundi, a small landlocked nation in Africa's Great Lakes region, emerged in 2006 from a brutal 13-year civil war.

In May, voters will go to the polls to elect a new parliament, with the presidential vote following a month later.

Nkurunziza has not yet confirmed whether he intends to attempt to try stay in power.

Thousands flee

UN rights chief Zeid Ra'ad Al Hussein on Wednesday warned that the country was 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 on 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 to clamp down on its activities.

The International Crisis Group (ICG) on Friday called the elections "decisive", warning in a new report that "tension is rising and prospects for free and fair polls are slimmer" by the day.

"Popular protests and the precedent set by the fall of Burkina Faso's president suggest street confrontations will take place if President Nkurunziza decides to impose his candidacy," the ICG think tank warned.

"The opposition wants revenge after its defeat in the 2010 polls, but it remains uncertain if its leaders will be allowed to contest the elections," it added.

Source: Agencies