A mass grave containing up to 30 bodies was shown to the public in the Burundian capital Bujumbura. Anti-government campaigners fighting against President Pierre Nkurunziza have been blamed for the killings, thought to have taken place at the height of the protests.
This is the first time authorities have shown a mass grave to the public though there have been several such claims in the past. Members of the press were also invited to the site in Mutakura, an anti-government neighbourhood.
Three bodies were exhumed from the burial site. Bujumbura's Mayor Freddy Mbonimpa, who was appointed by Nkurunziza, said one of the alleged killers has confessed that "there were about 30 bodies in the grave". The suspect had earlier turned himself in to the police.
"The murderer said they buried people there who had been killed for supporting a third term," added the mayor.
The grave is thought to date from April or May 2015 when there was widespread upheaval against Nkurunziza after he announced he would run for president a third time. The protests have snowballed ever since and continue in the landlocked eastern African nation.
Up to 400 people were killed during the clashes between anti-government groups and armed security forces. Amnesty International, citing satellite images, marked several sites as suspected mass graves in the country, blaming mostly Nkurunziza's troops.
The UN has ordered an investigation into the allegations. It has sent a three-member independent team of experts to examine the claims, which is expected to submit its findings by the end of March.