Nigerian Vice-president Yemi Osinbajo visited Monday the St. Francis Catholic Church where at least 22 people including children were killed.
The church in Owo town, southwest Nigeria was the scene of an attack Sunday when dynamite exploded inside the sanctuary and unidentified gunmen opened fire on worshippers attending mass.
The VP insisted they would be held accountable for their crimes: "As you know, the President has already ordered a full-scale investigation of these crimes and I assure you that without any doubt, those who perpetuated these will be found, they'll be fished out, they will pay for the consequences of this heinous crime."
The state government declared a seven-day mourning period and ordered the national flag to be flown at half-mast in Ondo.
According to the National Emergency Management Agency's local representative 40 people were wounded. The VP also visited some of them.
President Muhammadu Buhari condemned Sunday's "heinous killing of worshippers".
The UN Special Representative for West Africa and the Sahel, Mahamat Saleh Annadif, condemned what he called a "barbaric terrorist attack".
Pope Francis offered prayers for "the victims and the country, painfully affected at a time of celebration", and entrusted them both "to the Lord so that he may send his spirit to console them", a statement issued by the Vatican press office.
Vice-president Yemi Osinbajo lauded the national cohesion and the support, which poured in from across the country:"We see how the community has come together and how people from everywhere around the country have condemned this abominable act. Everyone: north, south. Everybody has condemned this. And this is a sore in the hearts of every Nigerian."
Nigeria's military is fighting a 12-year-long jihadist insurgency in the northeast and heavily armed criminal gangs. Since Christian communities have suffered in the past at the hands of jihadists in the northern region, Nigeria is often listed among the world's hot spots for anti-Christian persecution.
Nigeria's jihadist conflict has killed 40,000 and displaced two million more in the northeast.
Large-scale attacks in Nigeria's southwest are relatively rare.
Security will be a major challenge in the 2023 presidential race to govern Africa's most populous country and the continent's largest economy.