Human Rights Watch gave its estimate in a report that included testimonies from interviews with witnesses and with others who survived the riots.
Nigerian authorities have not given an official death toll for fear of provoking retaliatory violence.
But a government agency has said that more than 40,000 people in Africa's most populous nation were displaced by the riots. Some local organizations have said that at least 500 people were killed.
While international observers applauded Nigeria's legislative and presidential elections held in April, the violence that erupted in the aftermath threatened the stability of the oil-producing West African nation.
"The April elections were heralded as among the fairest in Nigeria's history, but they were also the bloodiest," said Human Rights Watch West Africa researcher Corinne Dufka.
The New York-based organization said it had conducted more than 55 interviews with witnesses in several states including Bauchi, Gombe, Kano, and Zamfara.
Interview were conducted with witnesses and victims of the violence, members of the clergy, traditional leaders, police officials and journalists.
Muslim rioters burned homes, churches and police stations after results showed that Goodluck Jonathan, a Christian from the south, had beaten his closest opponent Muhammadu Buhari, a Muslim from the north. Reprisal attacks by Christians began almost immediately.
The nation of 150 million people is divided between the Christian-dominated south and the Muslim north. The worst of the violence is believed to have been in the north-central state of Kaduna, where a large Christian community borders a Muslim one.
More than 300 people died in rioting in Zonkwa, a town in rural Kaduna state. A market in the town of Kafanchan was razed to the ground. The Human Rights Watch report says that in those parts of the state, most of the victims were Muslims caught in retaliatory attacks.
Many northerners believed someone from their region should be the next leader after the Muslim president died in office. Former President Umar Musa Yar'Adua had been expected to rule for another term, but his death catapulted Jonathan, his vice president, to power and left the presidency in the hands of a southerner before that region's turn. An unwritten agreement in the ruling party calls for its presidential candidates to rotate between the country's Christian south and Muslim north.
Jonathan last week created a panel to investigate the postelection violence. Its tasks include determining how many people were killed and the root cause of the wave of riots, but Human Rights Watch's report shows that some lack confidence in the panel.
"Going to these panels buys the government time and when the problem drops from the headlines, they go back to business as usual," the report quoted activist Innocent Chukwuma as saying.