A Reuters journalist saw the man's body on the ground with a bleeding head wound. It was unclear how he had died or who he was.
Kenya's Police Spokesperson Muchiri Nyaga said he was unaware of the casualty.
The death of 31-year-old blogger and teacher Albert Ojwang on June 8 came a year after more than 60 people were killed during protests initially sparked by proposed tax increases.
Police initially said he had committed suicide, but Kenya's police chief later apologised after an independent autopsy found that Ojwang's wounds pointed to assault as the cause of death.
As demonstrators took to the streets of Nairobi on Tuesday, police fired tear gas. Clashes also broke out when unidentified motorcyclists beat up protesters, dispersing them, the Reuters journalist said.
Local broadcaster NTV showed video of the bikers shouting "No protests."
Amnesty International's Kenya chapter, in a post on X, referred to the presence of dozens of motorbikes with two hooded passengers whipping protesters and members of the public.
Reuters could not immediately identify the bikers.
President William Ruto said last week that Ojwang had died "at the hands of the police" which he called "heartbreaking and unacceptable."
--Reuters--