Amnesty International called for an independent probe into the killings of civilians in Kenya after the disputed re-election of President Mwai Kibaki.
Political unrest across Kenya has left least 185 dead since Thursday's polling day.
"Amnesty International calls on the Kenyan government to establish an independent and impartial inquiry into the killings," said Erwin van der Borght, Amnesty's Africa Programme director on Monday.
The group highlighted deaths in the opposition strongholds of Kisumu town in western Kenya, in Nairobi's Kibera slum, in Mombasa "and elsewhere where people have been killed as a result of the post-electoral violence."
"Those responsible for human rights abuses should be brought to justice without undue delay," he added in a statement.
Amnesty urged the government and political parties to prevent further violence.
The state-funded Kenya National Human Rights Commission (KNCHR) led nine other local rights groups in urging the international community to pressure the Kenyan government to ensure that the election results are verified.
"We appeal to the international community to call for credible verification of the results before endorsing them," the Kenyan groups said in a joint statement.
"We also appeal to the police and security agencies to ensure that their responses are humane, measured and proportional to the level of threat.