The graves were discovered in Kilelengwani village, the epicentre of fighting that has killed 100 people in the last three weeks, including nine police officers.
The scale of the recent unrest has left many Kenyans convinced it was politically instigated and has raised fears of serious tribal fighting before elections next March.
President Mwai Kibaki imposed a curfew last week and sent extra security forces to the area to try to end the violence, intensified by an influx of weapons in the last few years.
“We don’t know yet whether they are attackers who died while in confrontation with security personnel, or were just victims killed by attackers during the clashes and buried,” regional police chief Aggrey Adoli said.
Settled Pokomo farmers and semi-nomadic Orma tribesmen have clashed for years over access to grazing, farmland and water in the coastal region. Dams along the Tana River, Kenya’s longest, supply about two thirds of the country’s electricity.
Police cordoned off the graves and said they had asked for a court order to exhume the bodies.
“We can’t know how many bodies are buried in the graves until we exhume,” Adoli said.