Police have searched the house of Burundi's main opposition spokesman, seizing computers and documents.
Burundi is emerging from a bitter civil war
Police sources say Pancrace Cimpaye is wanted for allegedly insulting the president. They went to arrest him, but he was not home when they arrived.
Mr Cimpaye accused the government of being behind weekend grenade attacks on the houses of five politicians.
The five were among 69 MPs urging the president to talk to the opposition about forming a unity government.
Burundi, one of the world's poorest nations, is emerging from a 12-year civil war.
Mr Cimpaye's younger brother, Parfait Karikumutima, told Reuters news agency that police arrived at dawn.
"They broke his (Cimpaye's) bedroom door with a drilling machine," he said.
Burundi's ruling party is locked in a power struggle, after its powerful chairman Hussein Radjabu was jailed and accused of trying to destabilise the country.
His supporters split from the ruling party and together with the opposition MPs have refused to pass bills presented by a government they refuse to recognise.
The BBC's Prime Ndikumagenge in the capital, Bujumbura, says the developments are adding to what is already a tense situation.
One rebel group remains outside the peace process and there are fears of a resumption of violence.