People in the country's central region found the heads and other remains after attacks on Sunday and Monday.
The media blamed the killings on Mungiki, a banned group that has fought weeks of battles with local minibus taxi operators who are resisting its demands for protection money.
With presidential elections due in the east African nation in December, many commentators suspected a political hand behind violence threatening the government's authority.
''(Mungiki) is out to demonstrate that it can operate and strike with impunity anywhere and everywhere,'' the Daily Nation newspaper said today in a front page editorial, below pictures of four of the six men who were decapitated.
''It is out to show the police and other government organs are feeble, helpless and unable to protect anyone who defies it.'' Police said they would hold a briefing later today.
Fear spread fast through the villages of Murang'a and Kiambu with some families fleeing the area as the victims' remains were discovered.
''I had gone out to answer a call of nature at around 3 am when I switched on my torch and saw the head of a human being placed on the roof of my chicken pen,'' Robert Kiunjuri, a teacher in Kianjogu village, told the Nation.
The 50-year-old victim's headless body had been dragged to the nearby home of a chief, where it was dumped at the gate.
Another head was found perched atop a telephone pole about a mile (kilometre) away, and another found after villagers heard two dogs fighting over it.
In neighbouring Kiambu, one head was left at a bus stop in the centre of the main town, local media said. A torso and three amputated legs were discovered in a ditch in a nearby village.
The victims all appeared to be local labourers and peasant farmers with no known links to the shadowy sect.
Mungiki, whose name means ''multitude'' in the local Kikuyu language, was banned in 2002 after members armed with knives and clubs killed more than 20 people in a Nairobi slum.
The group instils fear by promoting archaic Kikuyu rituals like swearing oaths, and many Kenyans believe it has been supported by corrupt politicians in the past.
''The police cannot claim to be seriously investigating Mungiki if they are not calling in for questioning such political leaders,'' the Nation said. ''Ultimately, the government must take full responsibility for failing to contain what is now clearly a national security issue.'' REUTERS SS KN1620