A police officer stands guard inside the St. Francis Catholic Church in Owo, Nigeria, on Monday. AP

The bloodshed at St. Francis Catholic Church in Owo town during a Sunday service was a rare assault in Nigeria's usually safer southwest and shocked a country grown used to militant attacks and mass kidnappings in the north.

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Blood still stained the church floor and debris lay scattered around a day after the attack that Ondo State governor's office said killed at least 21 people.

Gunmen hid among the worshippers inside the church and also opened fire on the congregation through the windows from outside.

St. Francis Catholic Church attack victim receives treatment in Owo, Nigeria, on Monday. AP

"Investigations revealed that some of the gunmen disguised as congregants, while other armed men who had positioned themselves around the church premises from different directions, fired into the church," the police statement said.

Fragments of explosives and three unexploded improvised devices were found at the scene.

Richard Olatunde, spokesman for the Ondo State governor's office, told AFP 21 people died after gunmen detonated dynamite inside the church before opening fire.

National Emergency Management Agency local representative Olanrewaju Kadiri said 22 people were killed, including several children, with another 40 people wounded.

No group has so far claimed responsibility for the attack and the motives were not immediately clear.

Vice President Yemi Osinbajo arrived on Monday to visit some of the wounded in Owo where he said perpetrators "will pay for this heinous" attack.

A victim of St. Francis Catholic Church attack receives treatment at St Louis Catholic Hospital in Owo Nigeria. AP

Survivors described panic as worshippers fled a sudden outburst of gunshots.

Father Andrew Abayomi, who conducted the service, told AFP some worshippers managed to close the church door and he escaped with others, including children, into the vestry to hide for about 20 minutes before emerging to the carnage.

"Even before leaving the church premises, I have seen some lifeless: I saw a woman shot beside the bus," he said.

Another survivor, Bade Salawu, recalled disbelief at hearing gunshots inside the church.

"They didn't come to steal anything, they didn't come to kidnap anybody... their aim was to just kill and destroy."

Another witness told AFP he saw at least five gunmen on the church premises.

The state government declared a seven-day mourning period for the victims, and ordered the national flag to be flown at half-mast in Ondo.

Agence France-Presse