A Nigerian human rights group says more than 500 people died after presidential elections earlier this month.
The Civil Rights Congress said the violence happened mostly in the northern state of Kaduna and that the number of victims could be even higher.
Rioting broke out when it emerged that Goodluck Jonathan, a southern Christian – had defeated a Muslim candidate from the mostly Islamic north.
Tens of thousands of people have fled their homes to escape the violence.
Mr Jonathan’s presidential rival Muhammadu Buhari has denied instigating the “sad, unfortunate and totally unwarranted” events.
The Civil Rights Congress said the worst hit area was the town of Zonkwa in rural Kaduna where more than 300 people died.
“The updated figure is about 516,” said Shehu Sani, head of the congress.
Correspondents say Nigeria is braced for possible further unrest over governorship elections on Tuesday in most of Nigeria’s 36 states.
Muslim opposition supporters staged riots on Monday when the results of the election became clear. Churches were set alight and Muslims were then targeted in revenge attacks.