The hacker group Anonymous stepped up its BART attack Wednesday, posting a partially nude photo of a man they say is the transit agency’s chief media spokesman, Linton Johnson.

Anonymous has been calling for the resignation of Johnson, who suggested that BART officials turn off cellphone service at underground stations earlier this month to thwart a protest against transit agency police.

The posting of the photo occurred as the BART board agreed to become the first public transit system in America to develop a policy regulating when to block cellphone service to protect public safety. It will take two to four weeks to craft a policy and discuss it with free speech advocates and BART’s citizen review committee, officials estimated.

BART has been under fire from free speech advocates since the Aug. 11 blackout, which disrupted communication between organizers of a protest against the officer-involved shooting of a homeless man.

After hearing from 18 speakers at a three-hour meeting, the board agreed that blackouts should only be allowed in very narrow circumstances.

About the time BART wrapped up its meeting, someone allied with Anonymous went on the attack in cyberspace, tweeting a photo purportedly showing Johnson partially pulling down his pants and exposing part of his genitals.

The photo appears to be in a social setting, and Johnson is shown looking into the camera with his arm around another person.

People commenting on an Anonymous online discussion group expressed sharp disagreement over whether to post the photo, with some saying the action was a “low road” to take.

BART officials called the posting an unethical invasion into the privacy of the transit system’s spokesman, who has repeatedly criticized Anonymous on behalf of the agency.

“It’s an unfortunate personal attack,” said BART Board President Bob Franklin, of Oakland. “Why shoot the messenger? Linton has done a good job of communicating BART’s position, but he doesn’t make policy.”

BART spokesman Jim Allison called the posting an illegal invasion of privacy, although he declined to elaborate. Franklin said it would be up to Johnson, not BART, to report the posting to authorities.

Johnson could not be reached for comment. He left word earlier this week that he was out of town on a family emergency.

This month, people associated with Anonymous hacked into the website of BART’s police union and posted home addresses, emails and passwords of 102 transit system police officers. The group also took credit for hacking into the myBART.org site, which is maintained by an outside vendor, and publishing home and email addresses of more than 2,000 riders who use the site for news about BART events and contests.

The FBI is investigating both hacking attacks.

Some Internet readers called the attack on Johnson a new low in tasteless online behavior.

“This just sends them in the trash can,” one commenter posted on the website of the San Francisco Weekly, which reported on the story.

“It’s tasteless and juvenile to release this pic,” said another reader. “The majority of folks in the Bay Area (according to a CBS 5 poll) support BART and their actions. … (Johnson) just represents how the majority of Bay Area residents feel, including myself.”

During the board meeting, an attorney with the American Civil Liberties Union said that once BART installed equipment to relay phone calls within underground stations, the phone system became a constitutionally protected forum. The agency cannot shut off cell phone service except under “exceptional” circumstances such as a hostage situation or a threat of a phone-activated bomb, said the attorney, Michael Risher.

Several BART directors agreed the system should draw narrow grounds for cell phone blackouts, but acknowledged it will take work to develop a policy.

“We need to defend First Amendment rights for people to protest,” said BART Director Lynette Sweet of San Francisco. “We can’t just sit back like Big Brother and spin it and say it’s all about safety.”

In defending the Aug. 11 cell phone blackout, BART top managers said Wednesday that they had learned that some protesters planned to chain themselves to areas of stations, while others planned to demonstrate on station platforms during peak use of those stations.

Overcrowding can create dangerous conditions if trains are delayed or stopped between 5 and 5:15 p.m. on weekdays when an average of 177 riders a minute pour into the Embarcadero station, said Paul Oversier, BART’s assistant general manager for operations.

“Delays by demonstrators holding doors or climbing into trains represent a real and tangible threat to the safety or our riders,” Oversier said.

When equipment problems delay trains in San Francisco during rush hour, it occasionally leads to rider panic attacks that require medical attention, he said.

Leaders of unions for BART train operators and controllers also described demonstrations on platforms as creating threats that riders may get shoved into oncoming trains or on the dangerous electric rail that powers trains.

Kristof Lopaur, a leader of the group No Justice No BART, accused BART of exaggerating the safety risks to justify cracking down on dissenters.

“Arresting people will not stop the protests,” he said. “We find it offensive and insulting to think you can define our limits.”

Lopaur said his group will not be satisfied until BART disbands its violence-prone police department.

Sweet and other directors said they will not abolish BART’s police department. Officers are needed to deter and investigate assaults, thefts and other crimes at stations and in parking lots, they said.

“We need a police force,” Sweet said.

Edward Hasbrook, spokesman for the Identity Project advocating free travel rights, said BART had no right to shut down cell phone service without approval of the Federal Communications Commission, the state Public Utilities Commission or a judge.

“I can’t just close my street” if a threat is feared, Hasbrook said. “I can go to a judge asking them to issue a restraining order.

Contact Denis Cuff

at 925-943-8267. Follow him at Twitter.com/deniscuff. Read the Capricious Commuter at IBAbuzz.com/transportation.