Details are still unclear. Wilson County Commission Albert Gamez told CNN the shooter died following a short police pursuit, but that it was not immediately clear whether the gunman was killed by officers or if his wounds were self-inflicted, Gamez said.
Wilson County Sheriff Joe Tackitt told the local paper there have been "multiple casualties and fatalities" and the shooter has been "taken down". But a law enforcement official who was briefed on the investigation said more than 20 people were killed and between 10 and 15 others were wounded, though the official stressed that the investigation was in its early stages and the figures could change.
President Donald Trump said he was monitoring the situation while in Japan on a 12-day Asian trip.
Another witness told CBS News that she heard about 20 gunshots when the shooter opened fire around 11:30 a.m. local time.
The incident is taking place at the First Baptist Church in Sutherland Springs, a small community south east of San Antonio.
Law enforcement officials works at the scene of a fatal shooting at the First Baptist Church in Sutherland Springs, Texas, on Sunday, Nov. 5, 2017.
Over 20 people were killed and more than 20 injured after a gunman entered the church in the small southeast Texas town and began firing.
Abbott tweeted: "Our prayers are with all who were harmed by this evil act".
Joseph Silva, 49, who lives about seven kilometres northeast of Sutherland Springs, said the police had instructed his family and neighbors to stay indoors.
Around 50 people usually attend the church, according to reports.
Within minutes of the gunfire, emergency personnel arrived at the scene, she said.