The six beheadings over the weekend came after weeks of simmering violence blamed on the outlawed Mungiki sect, which has been accused of extorting money from minibus drivers who provide the main form of public transport in Kenya.
The violence also has raised fears that Mungiki members are out to disrupt the general elections in December, when President Mwai Kibaki will seek a second term. Clashes have broken out every election year since 1992.
"The beheadings are an atrocious violation of human rights and above all an expression of disastrous politics in Kenya," said Mwambi Mwasaru, the executive director of Kenya Human Rights Commission. He said the sect shows the frustration and inequality in a country where more than 60 percent of the population live under the poverty line.
Mungiki is believed to have thousands of adherents, all drawn from the Kikuyu, Kenya's largest tribe. Members pray facing Mount Kenya, which the Kikuyu traditionally believed to be the home of their supreme deity. The sect also has encouraged respect for traditions like female genital mutilation and using tobacco snuff.
The group, whose name means multitude in the Kikuyu language, emerged in the late 1980s, apparently inspired by the bloody Mau Mau rebellion of the 1950s against British colonial rule. In recent years, the group has been linked to extortion, murder and political violence.
Mungiki was banned in 2002 after members killed more than 20 people in a Nairobi slum. The latest attacks took place in Muranga, 40 miles north of the capital, and Kiambu, 25 miles outside the city.
Eric Kiraithe, a spokesman for the national police, said no formal charges have been filed against the seven men, who were rounded up along with more than 200 others, most of whom have been released.