The New York-based group Human Rights Watch (HRW) says Nigerian police have used "excessive force" in the northern city of Kano.
Police say they were acting in self-defence
Police were deployed to restore order after riots between Muslims and Christians - sparked when hundreds of Muslims were killed in central Nigeria.
But HRW says officers may have unlawfully killed dozens of people.
It said it had seen several such cases in Nigeria, in which the perpetrators were never brought to justice.
"The police deployed to Kano were instructed to shoot on sight," said Peter Takirambudde, executive director of Human Rights Watch's Africa Division.
"In this volatile situation, the police should be attempting to provide security, not kill more people."
Citing "credible local sources", HRW highlighted an incident last Thursday, when police are said to have killed around 40 people by firing into a crowd.
The group also said the attackers in Kano did not use firearms. Yet many of the dead and injured had gunshot wounds.
Local sources believe these must have been inflicted by the police.
Police officials have said the police shot a number of people, but claimed that this was in self-defence.
More than 10,000 people have died in ethnic, religious and sectarian violence in Nigeria since the end of military rule five years ago.