Forty mentally ill patients have escaped from the Mathari Mental Hospital in Kenya's capital, Nairobi, after overpowering guards, police say, according to BBC News.

A search has been launched for the patients, some of whom are known to be violent, police told the BBC.

Kenya's Standard newspaper reports that the group escaped from the state hospital after complaining that the medicine given to them was ineffective.

Mathari is the biggest psychiatric hospital in Kenya. In 2011, rights groups called for an investigation into alleged human rights abuses at the hospital following a CNN documentary "Locked up and Forgotten".

The CNN crew reported that during a visit to the hospital, they found a dead body locked up in a seclusion cell with a patient.

Senior Nairobi police officer Moses Ombati told the BBC the patients had staged a protest, before overpowering guards and escaping.

The hospital reported the escape to police on Sunday, May 12 divisional police chief Samuel Anampiu is quoted by the Standard as saying.