At least 15 people have been killed in a surge of violence overnight in and around Kenya's capital, Nairobi.

At least three people were killed - and at least one beheaded - in the Banana district, where police have been cracking down on the outlawed Mungiki sect.

At least four people were killed when gunmen sprayed a bar in Kariobangi district with bullets.

And in another incident, police say they shot eight men who were allegedly attempting to rob the warehouse of an electric company.

The violence follows Thursday's sentencing of a former Mungiki leader. Maina Njenga was sentenced to five years in jail for illegal gun possession, and two years for illegal possession of marijuana.

Mungiki members are believed responsible for a string of gruesome slayings in the capital and central Kenya.

The sect was formed in the 1990s and banned in 2002 after a series of violent attacks. The group is believed to have thousands of followers drawn from Kenya's largest tribe, the Kikuyu.

London-based Amnesty International criticized police last week for what it called execution-style killings of 30 Mungiki sect members in a Nairobi slum.

Some information for this report was provided by AFP, AP and Reuters.