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.
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.
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.
In the northern city of Kano on Sunday, many Christians celebrated Easter in police and military barracks where they had taken shelter from the riots.
Eyo Anthony said he and his family fled when rioters set fire to shops in their neighbourhood.
“Although it has been calm in the past two days I don’t intend to go back to my house… until after the governors’ elections,” he said.
“I know how I managed to escape with my family and I don’t want to relive the same experience.”
Many in the north felt the next president should have been from their region because a Muslim president died last year before he could finish his term.
However, some analysts say the violence has more to do with poverty and economic marginalisation in the north than religion.
The north and south also have cultural, ethnic and linguistic differences.
Mr Jonathan was appointed to the presidency last year upon the death of incumbent Umaru Yar’Adua, a northern Muslim whom he had served as vice-president.
He has described the violence following the election as “horrific” and “shocking”.