ABUJA More than 800 people were killed and 65,000 displaced in three days of violence following a presidential election in April won by President Goodluck Jonathan, Human Rights Watch said on Monday.
Rioting erupted in cities in the mostly Muslim north after Jonathan, a Christian from the south, was declared the winner of the vote, deemed by observers and many Nigerians to have been the country's most credible for decades.
Jonathan's main rival, ex-military ruler Muhammadu Buhari, a Muslim popular in the north, has refused to accept defeat. His Congress for Progressive Change (CPC) party have gone to court to new polls in some areas.
"The April elections were heralded as among the fairest in Nigeria's history, but they also were among the bloodiest," said Corinne Dufka, West Africa researcher at the U.S. rights group.
"The newly elected authorities should quickly build on the democratic gains from the elections by bringing to justice those who orchestrated these horrific crimes and addressing the root causes of the violence," Dufka added.
Jonathan set up a panel of religious leaders, traditional rulers and lawyers last week to probe the violence.
Buhari's supporters say the riots in the days after the April 16 election -- in which homes, shops, churches and mosques were burned down -- was spontaneous but Jonathan's ruling People's Democratic Party (PDP) blames its opponents.
"The orchestrated violence that greeted the election especially in areas where the CPC won overwhelmingly was a direct fallout of inciting comments and directives made by their leaders even before the conclusion of elections," the PDP said in a statement on Sunday.
Jonathan won 59 percent of the vote compared to 32 percent for Buhari but although he picked up millions of votes in the north, Buhari was still ahead in almost all northern states.
(Editing by Angus MacSwan)