A gunman has opened fire in a Texas church as a Sunday service was being livestreamed online, killing two before an armed member of the congregation shot him dead.

The incident took place at the West Freeway Church of Christ in White Settlement, less than 10 miles west of Fort Worth.

A further two people were treated for minor injuries at the scene of the shooting, which took place during the 10am Sunday Service at the Texas church, with horrified viewers of the livestream witnessing the events unfold.

According to a church elder, the church member who was killed was a church security guard who had responded to the shooting. Church elder Mike Tinius said of the victim, "'He was trying to do what he needed to do to protect the rest of us. It's extremely upsetting to see anyone committing violence."

Tinius told a US publication that he did not know the gunman, and authorities have not yet publicly identified any motive for the shooting.

One witness described how the shooter approached a server during the communion and then started shooting.

One church member said on Facebook that a church deacon "who is a concealed carry instruction, and retired law enforcement officer, shot the guy before he could fire a third time!"

According to CBS 11 the deacon has been identified as a former FBI agent, and is also part of the church's security team.

The livestream of the incident shows the gunman opening fire and church members quickly ducking under the pews. However, within seconds about half a dozen church members rush towards the shooter with handguns pointed at him.

Texas Governor Greg Abbott has responded to the incident, saying: "'Our hearts go out to the victims and families of those killed in the evil act of violence that occurred at the West Freeway Church of Christ."

He added, "Places of worship are meant to be sacred and I am grateful for the church member who acted quickly to take down the shooter and help prevent further loss of life."

Texas Attorney General Ken Paxton has also said: "As reports come in, please pray for any victims and their families, this congregation, and the law enforcement officials at the scene. My office will assist in any way needed."