At least 50 people including children were killed, according to media reports, during the attack on St Francis Catholic Church in the town of Owo, which took place during Sunday mass as worshippers were celebrating the Christian holiday of Pentecost. Police have yet to release a death toll.
"Immediately they entered and started firing everywhere, so many people," said Alex Michael, who was shot in the leg while protecting his children by hiding them under chairs. He appeared dazed as he sat on his hospital bed on Monday.
Around him were other survivors, many with limbs wrapped in bloodstained bandages.
One man writhed and moaned on his bed, while a woman wept as she embraced her brother. A 15-year-old victim lay silently with a drip in his hand.
Dr Samuel Aluko, a registrar at the hospital, said 27 adult victims were receiving treatment in his department for a wide range of injuries, some of which were life-threatening. He said one woman had lost both legs.
Medical director Dr Ahmed Lasu said 13 children had been rushed to the hospital, of whom two were dead on arrival.
Owo is located in Ondo State in southwest Nigeria, a part of the country that does not usually experience violent conflict over religion. Authorities have said nothing about the identity or motive of the attackers.
The Catholic bishop of the diocese of Ondo, Jude Ayodeji Arogundade, said he had rushed to the church just after the attack.
"It was beyond what I ever imagined. A lot of bodies right there in the church, blood-soaked bodies," he told AIT television channel.
Pope Francis and Nigerian President Muhammadu Buhari were among those who expressed horror on Sunday.
Ondo State Governor Arakunrin Akeredolu on Monday directed that flags in the state should be flown at half-mast for seven days.
Vice President Yemi Osinbajo and former Lagos State governor Bola Tinubu, the frontrunners in the ruling party's primaries to select its presidential candidate for next year, both headed to Owo to offer their condolences.
Many shops in the town remained closed. Security forces were on the streets and helicopters passed overhead.
Southwest Nigeria is home to the Yoruba ethnic group, who are divided roughly equally between Muslims and Christians. The two communities cohabit peacefully.
An Owo resident said some local people were blaming the church massacre on members of the Hausa-Fulani ethnic group, who are predominantly Muslim and live mostly in northern Nigeria, with communities in other regions.
Neither the police nor state authorities have yet blamed any group.
There have been an increasing number of outbreaks of violence in recent years between Fulani herdsmen seeking land for their cattle to graze on and farmers from other ethnic groups seeking to shield their lands.
Population growth and increasing aridity in northern Nigeria are among factors that have driven Fulani herdsmen further and further south, increasing tensions over land use.
