Women and children were among 50 Catholic worshippers slaughtered during a "satanic" massacre at a church on Sunday.
Gunmen opened fire on the congregation outside and inside St Francis Xavue Catholic Church in Owo, Ondo state, Nigeria - slaughtering dozens.
At least 50 bodies, including those of women and kids, were taken to two nearby hospitals, a doctor at one of the facilities told Reuters.
Governor Arakunrin Oluwarotimi Akeredolu, who visited the church and the hospital treating the injured, branded the "vile and satanic attack" as "a great massacre" that should not be allowed to happen again.
The identity or motives of the attackers is not yet known.
President Muhammadu Buhari called the attack "heinous".
“No matter what, this country shall never give in to evil and wicked people and darkness will never overcome light,” he said.
In a statement, Rev Augustine Ikwu, a secretary of the Catholic Church in Ondo, said: "We turn to God to console the families of those whose lives were lost."
Rev Ikwu added: "It is so sad that while the Holy Mass was going on, unknown gunmen attacked St Francis Catholic Church...leaving many feared dead and many others injured and the Church violated."
Governor Akeredolu called the attack "the most dastardly act that could happen in any society" and a "black Sunday in Owo".
"The vile and satanic attack is a calculated assault on the peace-loving people of Owo Kingdom who have enjoyed relative peace over the years," he said.
"I commiserate with my people in Owo, particularly families of the those who lost their lives to this ugly and unfortunate attack."
Calling the shooting a "gruesome murder", Akeredolu urged people to "remain calm and vigilant".
"Do not take laws into your hands," he said. "I have spoken to the heads of the security agencies. I have equally been assured that security operatives would be deployed to monitor and restore normalcy to Owo kingdom."
Akeredolu vowed to make the attackers "pay for their crimes".
"Our people in Ondo state will not let down their guard," he said. "This will not happen again. I have urged the Heads of Security agencies to take all necessary steps."
Akeredolu said he had instructed the Commanders of the South West Security network to "convoke a joint security meeting of all Commanders in the South West with a view to commencing joint operations in the region immediately".
Nigeria is battling an Islamist insurgency in the northeast and, in the northwest, armed gangs carrying out attacks and kidnappings for ransom.
In the southwest, attacks like this are less common.
After the horror attack, medics said there was a pressing need for blood donations to treat the injured.
