The ethnic violence in Kenya following the disputed presidential elections last December was meticulously planned, says New York-based Human Rights Watch.
The election violence reignited old land disputes
Attacks on members of the Kikuyu community of President Mwai Kibaki were organised by local leaders, as were reprisals on rival groups, it says.
The lobby group calls for the prosecution of those responsible to help stabilise the country.
Some 1,500 died and 600,000 people were displaced during the violence.
The report also blames the police for using "excessive force" during opposition street protests.
Former opposition leader Raila Odinga has signed a power-sharing deal with President Kibaki - a move applauded by Human Rights Watch.
But the report's authors say further investigation is needed to determine the extent of the links between national leadership and those that carried out the violence.
"For the new government to function well and earn the people's trust, it needs to first heal the wounds by prosecuting those behind the violence," said Georgette Gagnon, Africa director at Human Rights Watch.
South African judge Justice Johann Kriegler is to start work this week on an official inquiry into allegations of electoral fraud in the election.
The report, based on the testimonies of about 200 witnesses, also backed plans for a truth and reconciliation commission to investigate.
The clashes exposed long, simmering tensions about power, land and distribution of wealth, says the BBC's Karen Allen in Nairobi.
In the Rift Valley, where Kikuyus were attacked, the violence was meticulously organised by local leaders, according to the authors of the report - Ballots to Bullets: Organised Political Violence and Kenya's Crisis of Governance.
But so too, were the reprisal attacks against non-Kikuyus - largely thought to be opposition supporters, claims HRW.
The violence in flashpoint areas such as Naivasha and Nakuru allegedly followed meetings with local businessmen and politicians from the president's Party of National Unity, which directed youths in their attacks.
The report accuses successive Kenyan governments of failing to address a culture of impunity and insists there should be no alternative to criminal prosecution for those responsible for the violence.
Although the report stops short of naming the perpetrators, it says they are well known in the community and are drawn from business and political circles.
Reacting to the report, Justice Minister Martha Karua said the government is committed to ensure that those found guilty of organising and participating in the violence face the full force of the law.
"Both parties, the Orange Democratic Movement and PNU have agreed that no one will be above the law, all perpetrators will be prosecuted," said Ms Karua, who is also a member of the negotiating team at the Kenya peace talks.