There is strong evidence that dozens of people have been buried in mass graves in and near the Burundian capital, Amnesty International said Friday.
Satellite images, video footage and witness accounts indicate that the bodies were buried in mass graves, according to the rights group.
"These images suggest a deliberate effort by the authorities to cover up the extent of the killings by their security forces and to prevent the full truth from coming out," said Muthoni Wanyeki, Amesty's regional director for east Africa.
Amnesty quoted residents of pro-opposition Bujumbura neighbourhoods as describing how police killed alleged government opponents and took the bodies to undisclosed locations in December.
Burundi has been gripped by violence since President Pierre Nkurunziza announced in April that he would seek a third term in office, despite the constitution setting a two-term limit.
Meanwhile, British photojournalist Philip Edward Moore and French journalist Jean Philippe Remy were arrested in Nyakabiga, one of the pro-opposition neighbourhoods, police said.
Le Monde said both journalists were on assignment for the French newspaper and demanded their immediate release.
Deputy police spokesman Moise Nkurunziza told dpa by telephone that Moore was with a group of "criminals" and fled with them on Thursday as police were approaching.
Police caught up with some members of the group, arresting the journalist and four other people, two of who were armed with pistols, according to the spokesman.
Remy then showed up at the place Moore had fled from, and was also arrested.
The fact that Moore fled made police suspect that the two had links with the "criminals," Nkurunziza said.
Burundian police often call government opponents criminals.
Both journalists were being questioned and would be released soon, if they were not found to be guilty of anything, the spokesman said.
More than 400 people have been killed in violent protests, clashes between police and government opponents, and attacks since April, according to the UN.