A leading rights group has accused Kenyan police of using "gruesome violence" to break up an opposition protest in the capital on Monday.
Protesters were beaten up even after being "subdued", said the Kenya National Commission on Human Rights.
Kenya's police chief said officers intervened to curb "lawlessness", but an internal inquiry would be held to look into the allegations against them.
The opposition called the protest to demand electoral reforms.
Church leaders appealed for calm, saying next year's general election should not lead to death and destruction.
"We must hold elections in a peaceful and harmonious manner without breaking this country," said Bishop Alfred Rotich of the Kenya Conference of Catholic Bishops.
A group of 15 opposition supporters pleaded guilty in court on Tuesday to taking part in the illegal protest in Nairobi, the capital.
They denied the more serious charge of being armed and breaching the peace.
Police fired tear gas and water cannon, and beat up opposition supporters with clubs in an attempt to break up the protest.
Police said they were forced to act to end rioting and looting.
"I condemn the lawlessness visited on the public by rioters yesterday and an internal inquiry is under way to determine whether any police officer broke any law while quelling the riots," said police chief Joseph Boinnet.
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 a statement.
The protest was called by main opposition leader Raila Odinga to demand that the electoral commission be dissolved, and that a new one be appointed.
Mr Odinga accuses the current commission of being biased, and fears that the elections will not be free and fair.
The commission denies the allegation.
Mr Odinga and President Uhuru Kenyatta are expected to be the main contenders in the election.
In 2014, the International Criminal Court dropped crimes against humanity charges against Mr Kenyatta, saying there was insufficient evidence to press ahead with the case.
Mr Kenyatta had been indicted in connection with post-election ethnic violence in 2007-08, in which 1,200 people died. He denied the charges.