"We have evidence that ODM politicians and local leaders actively fomented some post-election violence," said the lobby group's acting Africa director (Georgette Gagnon).
Human Rights Watch says members of President Kibaki's Kikuyu community were targeted following the announcement of his victory.
Their researchers spoke to member of the rival Kalenjin group, who said they were mobilised by their leaders to attack and loot Kikuyu-owned shops and businesses.
Local ODM officials and Kalenjin leaders "arranged frequent meetings following the election to organise, direct and facilitate the violence unleashed by gangs of local youth", HRW said.
One ODM official provided a lorry to ferry youths to attack Kikuyus, it said.
"If the leaders say stop, it will stop immediately," one Kalenjin elder said.