Garba Umar, police commissioner of Anambra state, said a gunman attacked St Philip Catholic Church early on Sunday.
But one parishioner, Uche Nonoso, told The Associated Press there were in fact two gunmen and more than 15 killed at the church.
The Reverend Hygi Aghaulor, communications director for the Nnewi Diocese, said the community was praying for the wounded.
No group has claimed responsibility for the attack, but the police say a manhunt has been launched.
Authorities said they did not believe Boko Haram was behind the attack.
The group has burned hundreds of churches over the past decade.
