Burundi's government on Saturday threatened a major crackdown after a week of political protests, accusing opposition and civil society groups of providing cover for "enemies of the state".

Burundi's security minister, General Gabriel Nizigama, said an overnight grenade attack that killed three people, including two police officers, was "linked to those who say they are demonstrating" against the controversial bid by the central African nation's president to serve a third term in office.

But in a sign of possible divisions, Burundi's defence minister declared the army's neutrality and called for an end to attacks on citizens' rights.

"The security forces will from now on take every necessary measure to stop and arrest these criminals. The police, with the army, will do everything to stop this uprising," the security minister said.

"From today we will no longer see demonstrations. We see criminals, militants and even enemies of the country."

Defence Minister General Pontien Gaciyubwenge, however, called on all sides "avoid any kind of undignified behaviour which could plunge the country back into the dark past."

At least 10 people have died and scores more have been hurt since the protests began last weekend. Nearly 600 people have also been arrested, according to police.

The political unrest erupted after the ruling CNDD-FDD party designated President Pierre Nkurunziza, a former rebel leader and born-again Christian from the Hutu majority, as its candidate in presidential elections due to be held on June 26.

Opposition figures and rights groups say Nkurunziza's attempt to stand for a third consecutive term violates the constitution as well as a peace deal that ended a civil war in 2006.

Nkurunziza has been in power since 2005. His supporters, however, say he is eligible to run again, since his first term in office followed his election by parliament -- not directly by the people as the constitution specifies.

- 'Detainees beaten' -

The protesters announced a two-day truce but warned they would return to the streets on Monday unless Nkurunziza backs down.

The United Nations voiced alarm over the political crisis, saying Nkurunziza's government was engaged in a brutal crackdown but "will not succeed in quashing dissent".

"According to one credible report, over 400 individuals are being held in extremely overcrowded conditions, with detainees having to sleep standing up," Rupert Colville, spokesman for the UN human rights office, told reporters in Geneva on Friday.

"Detainees have also been beaten, particularly on their feet and buttocks, with some of those released having trouble walking due to the beating," he added.

Social media has been blocked and radio stations have been shut down.

A top US envoy, who met with the president this week to try and convince him to back down, has also warned the situation remains "very dangerous".

The US has warned it could impose targeted sanctions against those in the heavily aid-dependent country who are behind the crackdown and violence against protestors.

There are fears that Burundi, which only emerged from civil war in 2006, could slide back into widespread violence.

The 13-year conflict left at least 300,000 people dead.
Source: AFP