Last week, incumbent Goodluck Jonathan was elected president of Nigeria but supporters of his Muslim opponent, Muhammadu Buhari, charged the election was rigged, Voice of America reported.
At least 500 people were killed when Buhari supporters attacked churches, homes and police stations, touching off reprisal attacks by Christians, the Civil Rights Congress of Nigeria said.
Police said an Easter Sunday explosion in Maiduguri was the work of the Islamic group Boko Haram that has been trying to establish an Islamic state in northern Nigeria for the past two years.
Jonathan increased security ahead of Tuesday's balloting to select governors.
Because of the violence, voting in the northern states of Kaduna and Bauchi has been delayed until Thursday.