A human rights group says election-related violence in Nigeria killed more than 800 people last month, when clashes broke out following the re-election of President Goodluck Jonathan.
New York-based Human Rights Watch said Monday that Nigerian authorities have failed to stop the recurring violence, and called for the government to investigate and prosecute those responsible.
The group said it has documented cases of police and the military using excessive force in trying to control rioting and sectarian violence.
It said the worst clashes happened in southern Kaduna state, where Muslim and Christian leaders told Human Rights Watch the violence left more than 500 people dead. A majority of those killed were Muslims.
Jonathan has formed a panel to investigate the violence, including determining how many people were killed, what led to the clashes and how to prevent future violence.
Much of the violence broke out in Nigeria's predominantly Muslim north, after it was announced that President Jonathan, a Christian from the south, won re-election on April 16.
Jonathan defeated his northern Muslim rival, former military ruler Muhammadu Buhari. Buhari has refused to concede defeat. His Congress for Progressive Change party says the voting process was flawed and has asked a court to throw out some of the poll results.
Nigeria has a population of 140 million that is divided roughly evenly between Muslims and Christians. The country has endured periodic sectarian violence, primarily in the central region where the two groups often live side-by-side.