Amnesty International says Kenyan police have used "unlawful force" against opposition supporters and bystanders after last week's election.
The human rights group on Monday cited cases of "police brutality" as well as violence and intimidation by backers of both opposition leader Raila Odinga and President Uhuru Kenyatta.
The group refers to violence in the Kenyan capital of Nairobi as well as Kisumu, the country's third-largest city and an opposition stronghold.
Kenyan government officials say opposition leaders have incited violence with incendiary rhetoric and that police have been attacked by mobs.
At least nine people have died in violence since the election Thursday. Some were shot by police; several died in fighting between different ethnic groups.