Red-faced Kenyan police on Tuesday arrested eight of their own officers and launched a manhunt after a man accused of murdering and dismembering 42 women escaped from a Nairobi police cell along with a dozen other people.

The escapees include Collins Jumaisi, 33, described by police as a "vampire, a psychopath", who was arrested last month after the horrific discovery of mutilated bodies in a dump in a slum of the Kenyan capital.

Jumaisi and the 12 others, all Eritrean nationals, appeared to have escaped by cutting through a wire mesh roof at the police station.

"Our preliminary investigations indicate that the escape was aided by insiders," acting national police chief Gilbert Masengeli said in a statement.

He told reporters that eight officers on duty at the time have been suspended and "placed in custody", adding that the matter was being investigated by the Internal Affairs Unit.

Police said they discovered the breakout when officers made a routine visit to the cells at around 5 am to serve the prisoners breakfast.

"On opening the cell door, they discovered that 13 prisoners had escaped by cutting the wire mesh in the basking bay," police said, referring to a covered courtyard area in the station where detainees could get access to fresh air.

The 12 Eritreans had been arrested for being in Kenya illegally, police said, adding that four others who did not escape were helping with the investigation.

The police station is located in the upmarket Nairobi district of Gigiri, home to the regional headquarters of the United Nations and numerous embassies.