The bomber struck shortly after noon in front of a church in the town of Owo.
At least 28 people were killed, Olemi said.
“The attackers came on motorcycles and started firing from time to time,” he said. “They killed many people inside the church.”
The victims are being taken to the Federal Medical Center in Owo, Olemi said.
The state police could not confirm the total number of casualties at St. Francis Catholic Church, a police spokesman told CNN, or they could not identify those behind the attack.
Ondo Governor Aragunrin Agarotolu said he was “shocked” by the attack and called it “Black Sunday in Owo.”
“I am deeply saddened that the innocent people of Owo who worship at St. Francis Catholic Church today were attacked and killed without provocation,” he said on Twitter, “the brutal and satanic attack is a calculated attack on peace. The loving people of Owo Kingdom have been enjoying peace for many years.”
The governor promised to “dedicate all available resources to hunting down these attackers and paying them.”
“We will never bow to the machinations of heartless elements in our resolutions to eradicate criminals,” he continued, urging the public not to “take the laws into your own hands”.
“I urge our people to remain calm and vigilant. Do not take the law into your own hands. I have spoken to the heads of the security agencies. I assure you that security personnel will be sent to monitor the Ovo Kingdom and restore normalcy,” Agoredol wrote.