Burundi police said Friday they had arrested a key opposition leader in connection with a five-day battle earlier this month between the army and gunmen in which some 100 insurgents were killed.

Patrick Nkurunziza, president of the youth wing of the Front for Democracy (Frodebu) party, was arrested on Thursday, police spokesman Pierre Nkurikiye told AFP.

He is the latest in a series of politicians to face criminal charges ahead of key polls in May and June, which the opposition say is the government's way to block them from running for seats.

Also on Thursday, Frodebu deputy president Frederic Bamvuginyumvira was sentenced to five years in jail for bribery, which he claimed was handed down to "remove me from the electoral race".

READ: Burundi opposition chief sentenced to five years for bribery

Nkurunziza, also the youth head of the main opposition coalition, Democratic Alliance for Change (ADC), is the most prominent figure arrested in connection with a rebel force that entered the country from neighbouring eastern Democratic Republic of Congo.

He was arrested on a warrant for "complicity with the armed bandits", police said, without giving further details.

The army say documents they seized last week showed the insurgents had planned a major offensive to destabilise the country.

READ: Rebels 'planned offensive' ahead of Burundi elections

Burundi, a small nation in central Africa's Great Lakes region, emerged in 2006 from a brutal 13-year civil war and its political climate remains fractious ahead of the polls.

President Pierre Nkurunziza, in power since 2005, is expected to run for a third term in office despite opponents' claims that a new mandate would violate Burundi's constitution.

READ: US opposes Nkurunziza third term in this year’s poll

Frodebu party spokesman Phenias Niyigaba said Nkurunziza had also been questioned about links with Alexis Sinduhije, the exiled president of the opposition Movement for Solidarity and Development (MSD).

While officially the army says it does not know who the rebels are, senior military officers point the finger of blame to Sinduhije, who is now based in Belgium.

READ: Burundi opposition chief charged with rebellion

Both Frodebu and Sinduhije's MSD are part of the ADC coalition.