Police fired tear gas and water cannon, and beat up opposition supporters with clubs in an attempt to break up the protest.
The Kenya National Commission on Human Rights said protesters were beaten up even after being "subdued” at the protests demanding electoral reform on Monday.
In a statement the commission said there were “gory scenes” of violence at the protest, which was in serious violation of human rights.
The opposition called for the protest, which was deemed illegal and 15 people pleaded guilty in court on Tuesday to taking part in the protest, though they denied the more serious charge of being armed and breaching the peace, the BBC reports.
Kenya's police chief Joseph Boinnet said officers intervened to curb "lawlessness", but an internal inquiry would be held to look into the allegations against them.
Police fired tear gas and water cannon, and beat up opposition supporters with clubs in an attempt to break up the protest, the BBC reports.
However, the Kenya National Commission on Human Rights said the protesters were subjected to "serious bodily harm", even after they had been "subdued".
"When police disobey the law with such corrosive impunity, they lose legitimacy as law enforcers and alienate themselves from the very public they are mandated to serve," it added in the statement.
He accused the current commission of being biased, and fears that the elections will not be free and fair.
Kenya does not hold its next presidential and parliamentary polls until August 2017.
Odinga and President Uhuru Kenyatta are expected to be the main contenders in the election.