Gunmen killed several churchgoers in Ondo state, south-west Nigeria.
President Muhammadu Buhari said “only fiends from the nether region” could have done this “dastardly act”.
The armed men entered St Francis Catholic church in the town of Owo and fired at the congregation during a service to mark the holiday of Pentecost, witnesses said.
Nigeria has experienced an upsurge in violence in recent months.
Kidnappings and attacks have been reported across the vast country.
The church has denied initial reports that the priest had been abducted.
No figures for the numbers killed in the attack have been officially confirmed.
But a doctor at a local hospital, quoted by the Reuters news agency, said that at least 50 bodies had been taken to two hospitals in the town.
In a series of tweets, Ondo state Governor Rotimi Akeredolu called it a “vile and satanic attack” on innocent people . He appealed for calm urging people not to take the law into their own hands.
“The assailants will be hunted down and they will pay for their crimes,” he added in a message sent after being briefed at the scene.
Pope Francis was praying for the victims who had been “painfully stricken in a moment of celebration”, according to a statement from the Vatican.
“No matter what, this country shall never give in to evil and wicked people,” President Buhari said in a statement. He is in the final year of his two-term presidency and has been criticised for failing to get to grips with the country’s security problems.