An effort to shut down San Francisco’s juvenile hall ran into unexpected opposition as it was introduced Tuesday, with the head of the local NAACP lambasting the plan as “pure politics.”

Eight San Francisco supervisors — a veto-proof majority — introduced the legislation that would close the mostly empty facility within three years and create community-based alternatives, including lockdown environments for youths who require a secure placement.

But the Rev. Amos Brown, president of the San Francisco chapter of the NAACP, called the idea a “knee-jerk political move,” and decried the fact that the supervisors didn’t engage more community members when crafting the measure.

“That doesn’t bode well for creating community,” he said. “It’s rugged individualism.”

At a boisterous noon rally, dozens of public officials, community activists and formerly incarcerated youths spoke out in support of the closure, including a majority of the 11 members of the Board of Supervisors, District Attorney George Gascón and school board President Stevon Cook.

As the speakers passionately called on the city to shut down juvenile hall, the audience chanted, applauded and whistled in support. Some held up signs that said “Shut it down!” and “No child belongs in jail!” and “Education instead of incarceration!”

“This jail for kids is morally repugnant,” said Supervisor Matt Haney, a sponsor of the proposal. “It’s outdated, it’s ineffective, and anyone who says that we shouldn’t set a date to shut it down is on the wrong side of history.”

The supervisors drafted the legislation in response to a recent Chronicle report that documented a dramatic drop in serious youth crime that has left the state’s juvenile halls nearly empty and costs per inmate skyrocketing.

Supervisors Shamann Walton, Hillary Ronen, Gordon Mar, Aaron Peskin, Sandra Lee Fewer, Vallie Brown and Ahsha Safaí are the other co-sponsors of the legislation. With eight of the 11 members signed on, the board has a veto-proof majority. Public Defender Mano Raju has also endorsed the measure, as have Leif Dautch and Chesa Boudin, two of the three candidates running for district attorney.

Speakers at the rally called on city officials to create more programs to support incarcerated youths, rather than locking them behind bars.

“Juvenile hall did not support me. I knew what I needed, and no one ever listened,” said Tenaya Jones, 18, who said she was put in juvenile hall when she was 14 years old. “We need more programs … where young women and men can come and grow and plan for their futures.”

Brown and other critics said they feared that without juvenile hall, the county would have to send youth offenders to other counties or states, which the legislation’s authors said was not the case.

Dan Daniels, coastal area director of the NAACP — one of the few public commenters opposed to the measure at Tuesday’s Board of Supervisors meeting — said closing down juvenile hall is not the solution. He said youth offenders would just get pushed to facilities outside the county. Excess money being spent on juvenile hall, he said, should instead be spent on youth job programs and mental health services.

“When a kid goes into juvenile hall, sometimes that is the only time they have access to medical treatment,” he said. “Closing the hall is not the answer.”

The measure would establish a working group to implement the closure, and create non-institutional alternatives for youths who don’t need to be in a lockdown facility, as well as a secure rehabilitative facility in San Francisco for the relatively small number who do.

A final vote on the measure could come as early as June. Mayor London Breed, who recently created a community task force on juvenile justice reform, has not taken a position on the measure.

In its investigation, The Chronicle found that San Francisco’s juvenile hall is two-thirds empty on a typical day and the average annual cost to incarcerate a child has reached $280,000, up from $135,000 in 2011.

Counties across California are reporting similar trends, with nearly every juvenile hall in the state less than half full and costs creeping up as high as $500,000 per child each year.

Brown’s NAACP counterpart in Alameda County, George Holland Sr., said he supports the idea of shutting down juvenile halls, although he fears that poor policymaking could make things worse than they already are.

“I’m all for closing down juvenile halls,” said Holland, a former juvenile probation officer. “But I need to know what the alternative is.”

Brown acknowledged that there were concerns surrounding the conditions at juvenile hall, “but it’s not a major issue.”

He said San Francisco’s juvenile hall isn’t the harsh institution it once was, offering more support and educational services, including interaction with the faith community inside the facility.

Brown also criticized the supervisors for failing to consult with the faith community and civil rights leaders before drafting the legislation.

Walton, lead author of the measure, said he spent time in the juvenile hall as a youth and visited the same site three weeks ago. It hasn’t changed, Walton said.

“Youth still walk in a line. Youth still sleep on a mat with a concrete slab,” he said. “Bottom line: Incarceration doesn’t work. It didn’t work for me. Mentoring programs led me to success, not being behind bars.”

The supervisors have been working with policymakers and community organizations while drafting the legislation, Walton said.

“I don’t need permission from Amos Brown to do my job,” he said. “My community is definitely behind this.”

Jill Tucker and Trisha Thadani are San Francisco Chronicle staff writers. Email: jtucker@sfchronicle.com, tthadani@sfchronicle.com Twitter: @jilltucker, @trishathadani