Adelegbe Timileyin, a federal lawmaker representing the Owo area, told The Associated Press that at least 50 people had been killed.
The attack came amid renewed social and economic tensions in Nigeria, where regular killings and kidnappings have compounded a deep sense of insecurity and resentment of the government ahead of the next presidential election scheduled for February.
As Mass was taking place around 11:30 a.m., armed assailants shot at worshipers from outside the church while other gunmen targeted people inside the building, the police said in a statement on Sunday evening.
“It is a black Sunday in Owo,” Ondo’s governor, Arakunrin Akeredolu, said, condemning a “vile and satanic attack” against people “who have enjoyed relative peace over the years.”
Nigeria is roughly divided between Christians living predominantly in the south and Muslims populating the country’s north.
Much of the violence plaguing Nigeria, such as killings and kidnappings, have mostly occurred in the country’s northwest and center.
Last month, gunmen killed dozens of people in the central state of Plateau, and in April eight people were killed and dozens were kidnapped on a popular train route connecting the capital, Abuja, to the regional hub of Kaduna in the north.