At least 22 "evil-doers" have been killed in clashes between Burundi's security forces and an armed group near the country's economic capital, Bujumbura, according to police. The fighting on Sunday in the hills of western Bujumbura-Rural province also killed two police officers, police spokesperson Pierre Nkurikiye said late on Tuesday. More:More than 6,000 bodies found in Burundi mass gravesBurundi's president vows to step down at term's endWill Burundi's 2020 elections be free and fair?Nkurikiye said the group "took advantage of the electoral period" to cause trouble in the country, which is scheduled to hold a presidential election in May. A teacher who led the group was among those killed in the week-long operation in the districts of Isale and Nyabiraba, he added. Local sources, however, told The Associated Press news agency that at least 12 people were executed on Sunday in Nyabiraba after being arrested by police and the governing party youth group, known as the Imbonerakure. They spoke on condition of anonymity out of concern for their safety.Meanwhile, Burundi's main opposition party, the National Freedom Council (CNL), said it saw a link between the incident and the May 20 vote. President Pierre Nkurunziza, in power since 2005, shocked the country by announcing he would not seek re-election after a constitutional change allowed him to do so.His controversial election to a third term in 2015 plunged the country into crisis.The violent aftermath of the last presidential election in 2015 made Burundi a focus of an investigation by the International Criminal Court for alleged murder, rape, torture and disappearances. The government denies the accusations.Civil unrest killed hundreds of people and drove 400 000 from their homes.