Two people including a five-year-old boy were wounded after a woman opened fire at celebrity US pastor Joel Osteen's megachurch in Houston, Texas, one of the largest religious venues in the United States, police said on Sunday.

The woman was armed with a long gun when she entered the non-denominational Christian church with a child just before 2:00 pm (2000 GMT), Houston police chief Troy Finner told a press conference.

Arriving at the church, housed in a converted former sports arena which can seat some 16,800 people, she began to fire, Finner said -- but off-duty police officers working security at the venue quickly shot back and she was killed.

A "five-year-old kid was hit and is in critical condition at our local hospital," Finner said, while a man in his 50s "who didn't have anything to do with it, I don't think" is being treated for a gunshot wound to the leg.

"When I entered the sanctuary I saw people running and saying: they are shooting, they are shooting, and we could hear gunshots," one witness told local television station KHOU.

"We tried to open some doors in the hallway but they were locked. Some people went out to get to their cars, but others like me did not know where to go."

A church volunteer hid them in the bathroom, piling furniture up against the doors and warning them not to move, she said.

"There were children on the third and fourth floors, the parents were worried about the children. We put things in God's hands," she added.

Mass shootings are common in the United States, where there are more guns than people and about a third of adults own a firearm.

Polls show a majority of Americans favor stricter gun regulations, but the powerful gun lobby and mobilized voters supporting the country's culture of strong gun rights have repeatedly stymied lawmakers from taking action.

Osteen, whose services are broadcast live to millions weekly and who has largely sought to keep his church apolitical, said he was "devastated" by the attack, and described himself as "kind of in a fog."

The shooting began between services, just before the start of a Spanish-language service, the 60-year-old said, adding: "I can only imagine if it would have happened during the 11 o'clock service... we thank God for that."

"We don't understand why these things happen, but we know God's in control... There are forces of evil but the forces that are for us, the forces of God, are stronger than that," he told a press conference, adding that he would pray for the victims as well as the shooter.