A human-rights organisation has claimed that Kenyan police killed as many as 8,040 people by execution or torture during a crackdown on a banned sect.
Police carried out a major operation against Mungiki members in June
The group said a further 4,070 people had gone missing as security forces tried to wipe out the Mungiki sect.
The deaths and disappearances occurred over five years up to August 2007, said the Oscar Foundation Free Legal Aid Clinic-Kenya.
A Kenyan police spokesman has dismissed the report as "fictitious".
"The people disseminating it have a questionable character and motive," Eric Kiraithe told the Associated Press news agency.
Banned in 2002
Thought to be ethnic Kikuyu militants
Mungiki means multitude in Kikuyu
Inspired by the Mau Mau rebellion of the 1950s
Claim to have more than 1m followers
Promote female circumcision and oath-taking
Believed to be linked to high-profile politicians
Control public transport routes, demanding levies
Blamed for revenge murders in the central region
The report said Kenya's General Service Unit carried out the killings during operations in slum areas.
The document was based on interviews with relatives, autopsy reports, and police and other records.
It comes shortly after the Kenya National Commission on Human Rights linked police to the execution-style deaths of nearly 500 Mungiki in a crackdown on the sect carried out over the last five months.
The police have said criminals are responsible.
The allegations of extrajudicial killings are being made at a politically sensitive time, just weeks before Kenya's presidential elections.
Police moved against the Mungiki after they terrorised parts of the Kenyan capital Nairobi and the centre of the country earlier this year.