A suspect collapsed and died at the Matopeni police post in Kayole, Nairobi.
The suspect had been arrested for being drunk and disorderly on Sunday night.
He had been taken to the police station for processing when he collapsed and died.
The suspect has not been identified.
Police said at the post, there were more than 50 people who had been detained for various crimes.
According to police, the officers on duty responded to the collapse and tried to resuscitate him in vain.
He was rushed to Mama Lucy Hospital where officials confirmed him dead.
Police said an identification and postmortem exercise is planned to establish how and why he died.
Police informed Ipoa of the Sunday night incident.
The Independent Policing Oversight Authority is investigating the circumstances under which the suspect died.
The agency is mandated by law to investigate such incidents and make recommendations to the Office of Director of Public Prosecution.
Officials want to ascertain whether the man died of natural causes or police negligence is to blame.
Elsewhere, police said they are investigating how a one-month-old baby died in a house in Kangemi, Nairobi.
The baby identified as Emmanuel Abraham was found unresponsive in the house and it is not clear what caused his death.
The mother of the child told police she had stepped out of their house and when she came back she found him unresponsive.
The body was moved to the Chiromo Mortuary pending an autopsy.
In Kawangware, the body of Anthony Mugambi, 33, was found in his house, long after he had died.
It is suspected the man collapsed and died in the house where he lived alone.
The cause of death is yet to be known but police said they are investigating.
The body was discovered on Sunday night and moved to the mortuary.