NASHVILLE — Police and security guards keep watch as thousands of children zigzag through Nashville’s downtown bus hub each morning and afternoon, catching buses between home and school.

Barring some court challenge this month, police won’t be alone in carrying lethal firepower through the Music City Central station.

A law signed by Governor Bill Haslam that takes effect July 1 will force Nashville to let people carry loaded guns in the station and potentially even on the city buses thousands of students ride each day.

The law gives local governments an ultimatum: Either let handgun permit-holders carry guns, or invest in metal detectors and security guards to inspect bags wherever people enter public facilities.

But transit officials say it’s logistically impossible to add metal detectors at stations and secure all city buses. Even trying to apply airport-style security to a public transit system would cost millions and create commuter chaos, they say.

The law has officials in Tennessee’s large cities fearing for children’s lives, but Republican state lawmakers who approved the change in a largely party-line vote said the law enables people to properly defend themselves.

The policy is another example of conservative lawmakers from rural districts restricting more liberal policies in urban areas.

Nashville school board chairwoman Anna Shepherd said the law puts ‘‘the NRA ahead of the safety of our students, and that’s just not right.’’