The bloodshed occurred at St. Francis Catholic Church in the town of Owo in Ondo state, more than 200 miles southwest of Abuja, the Nigerian capital. A group of unknown gunmen stormed the church at around 11:30 a.m. local time during a service for Pentecost Sunday, with about four of the assailants opening fire inside the building while others shot at worshippers outside, according to Funmilayo Ibukun Odunlami, a spokesperson for the Nigeria Police Force's command in Ondo state.
"Some lives were lost and some sustained varying degrees of injuries," Odunlami said in a statement on Sunday, later telling ABC News on Monday that police do not yet have an estimate on the number of casualties.
Health workers at the Federal Medical Center in Owo told ABC News on Monday that at least 35 bodies had been transported to the hospital from the scene of Sunday's attack. They said there was also an urgent need for blood donations for the many wounded.
Meanwhile, the Catholic Laity Council of Nigeria said in a statement Monday that "more than 50 parishioners" had died and the gunmen were "suspected to be bandits."
The attack remains under investigation and specialized police units have been deployed to Owo "to restore normalcy and fortify the entire community," according to Odunlami. A motive for the massacre and the identity of the perpetrators was not immediately clear, as no group has claimed responsibility.
A U.S. official briefed on the situation told ABC News that the attack reportedly began with an explosion outside the church, followed by gunmen shooting sporadically at worshippers inside. Stray bullets also killed passersby, the official said.
One of the priests at the church, Rev. Fr. Andrew Abayomi, said they were at the end of the service and people were getting ready to leave when the first gunshot rang out.
"We hid inside the church but some people had left when the attack happened," Abayomi told reporters in Owo on Sunday. "We locked ourselves in the church for 20 minutes. When we heard that they had left, we opened the church and rushed victims to the hospital."
A Catholic Church spokesperson in Nigeria, Rev. Fr. Augustine Ikwu, said the attack "has left the community devastated," but that all the priests and bishops in the parish were "safe."
"Nonetheless, let us continue to pray for them and the good people of Owo and the state at large," Ikwu said in a statement on Sunday.
Ondo state Gov. Arakunrin Oluwarotimi Akeredolu, who visited the scene of the attack as well as some of the hospitalized victims, was "deeply saddened by the unprovoked attack and killing of innocent people of Owo," according to his spokesperson, Richard Olatunde.
"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," Olatunde said in a statement on Sunday. "We shall commit every available resource to hunt down these assailants and make them pay."
Nigerian President Muhammadu Buhari condemned what he described as "the heinous killing of worshippers."
"No matter what, this country shall never give in to evil and wicked people, and darkness will never overcome light," Buhari said in a statement on Sunday.
ABC News' Josh Margolin contributed to this report.