Gunmen opened fire on worshippers at a Catholic church in south-western Nigeria on Sunday and detonated explosives, officials said.
Dozens are feared dead, including children.
Local politician Ogunmolasuyi Oluwole said the attackers targeted the St Francis Catholic church in Ondo state just as worshippers gathered on Pentecost Sunday.
Among the dead were many children, said Mr Oluwole, who visited the scene and also the hospital where many of the wounded were being treated.
While much of Nigeria has struggled with security issues including Islamic extremism, Ondo is widely known as one of Nigeria’s most peaceful states.
Nigeria’s president, Muhammadu Buhari, said “only fiends from the nether region could have conceived and carried out such a dastardly act”, according to a statement from his spokesman.
“No matter what, this country shall never give in to evil and wicked people, and darkness will never overcome light. Nigeria will eventually win,” said Mr Buhari, who was elected after vowing to end Nigeria’s prolonged security crisis.
Adelegbe Timileyin, who represents the Owo area in Nigeria’s lower legislative chamber, said the presiding priest was abducted as well.
“Our hearts are heavy,” Ondo governor Rotimi Akeredolu tweeted on Sunday. “Our peace and tranquility have been attacked by the enemies of the people.”
Authorities did not give an official death toll. Mr Timileyin said at least 50 people had been killed, though others put the figure higher.
Videos appearing to be from the scene of the attack showed church worshippers lying in pools of blood while people around them wailed.