The world was shocked at the bloodshed in Kenya, previously seen as a haven of stability on a volatile continent, and many leaders helped pressure Mwai Kibaki, the president, and Raila Odinga, the opposition leader, into a February 28 power-sharing pact.
Human Rights Watch accused police of causing "hundreds" of deaths by using excessive force during the two-month crisis in the East African country, especially in opposition strongholds like the town of Kisumu.
Fleeing children had been shot, the group said in its 88-page report.
Lethal force was used quickly in opposition areas but restraint was shown towards pro-government supporters, it said.
The crisis was Kenya's worst since independence from Britain in 1963 and damaged its reputation as a prosperous trade and tourism hub.
Kenya is East Africa's biggest economy.
Human Rights Watch blamed successive post-independence governments for failing to address land and poverty issues at the root of the violence.
"Much of the ethnic-based violence was organised by local leaders, politicians and businessmen from all sides, according to eyewitnesses," it said.