Goodluck Jonathan, who assumed office last year after the death of President Umaru Yar'Adua, received more than half of the votes cast in an April election, beating his primary challenger Gen. Muhammadu Buhari by about 10 million votes, an independent election commission determined.
Riots broke out in the predominantly Muslim north after news that Jonathan, a Christian, defeated Buhari, a Muslim.
Human Rights Watch said Monday it had evidence to suggest more than 800 people died in post-election violence and another 65,000 were displaced.
Corinne Dufka, a researcher for West Africa at Human Rights Watch, called on Nigerian authorities to investigate reports of violence by mobs and state actors alike.
"The April elections were heralded as among the fairest in Nigeria's history but they also were among the bloodiest," she said in a statement. "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."