Protesters roughed up police spokesman Charles Owino demanding he takes responsibility for the murder of a human rights lawyer and two others.
Owino was being driven in a police car from Vigilance House when the demonstrators spotted him at Freedom Corner on Monday.
They blocked the road, demanding that he alight and join their march to IG Joseph Boinnet’s office.
On alighting, they poured red paint on the spokesman and forced him to carry a coffin written “stop extra judicial killings”.
When they reached the Inspector General’s office, the crowd refused to be addressed by Owino and demanded Boinnet’s presence.
Protesters proceeded to dump coffins outside the IG’s gate after they were informed that he was in another meeting.
More than 30 local and international human rights groups and lawyers took to the streets to protest the murders of lawyer Willie Kimani, his client Josephat Mwenda and taxi driver, Joseph Muiruri.
Activists demanded that government agencies begin investigations into the “heinous” deaths.
The demonstrations also took place at Mombasa law courts, Jomo Kenyatta Sports Ground in Kisumu and Railways Grounds in Nakuru.
Protesters carried placards demanding “accountability for victims of police executions”, and calling to “prosecute those responsible for death of Willie, Joseph and Josephat”.
They marched the area with red-stained t-shirts as they chanted.
Three officers linked to the deaths – Fredrick Leliman, Stephen Chebulet and Silvia Wanjiku – have been detained for 14 days to allow investigations.
Kimani, 32, and his client Mwenda, a boda boda operator, had filed a complaint alleging that Mwenda had been shot and injured by police in April 2015.
Mwenda was then charged with a range of offences, including possessing drugs, gambling in public and resisting arrest, the activists said.
The two and taxi driver Joseph Muiruri went missing after a court session at Mavoko law courts on June 23.
At about 5.30 pm last Thursday, their bodies were later found floating in Oldonyo Sabuk River .