SAN FRANCISCO — A secretive, international group of hackers has aimed its keyboards and social media masses at BART, threatening to retaliate against the transit agency for its decision to cut off cellphone service at a planned protest Thursday.

The activist group, Anonymous, which has targeted everyone from dictatorships to the Church of Scientology, has planned a “peaceful protest” at 5 p.m. Monday in the Civic Center station, asking participants to wear “blood” stained shirts in remembrance of a homeless man who was killed at that station.

The group issued a statement to BART: “We will set those who have been censored free from their silence. That’s a promise. Anonymous demands that this activity revolving around censorship cease and desist and we know you are already planning to do this again. We will not issue any more warnings.”

The group also published the numbers and email addresses of BART directors, encouraging members to phone, email and “black fax” the agency.

The question of how BART handled the Thursday protest resonated with others as details emerged of agency officials’ decision to cut off underground cellphone service for a few hours at several stations. Commuters at stations from downtown to near the San Francisco Airport were affected as BART officials sought to tactically thwart the planned protest over the recent fatal shooting of a 45-year-old homeless man by transit police.

Two days later, the move had civil rights and legal experts questioning the agency’s move and drew backlash from one transit board member who was taken aback by the decision.

“I’m just shocked that they didn’t think about the implications of this. We really don’t have the right to be this type of censor,” said Lynette Sweet, who serves on BART’s board of directors. “In my opinion, we’ve let the actions of a few people affect everybody. And that’s not fair.”

Similar questions of censorship have arisen in recent days as Britain’s government put the idea of curbing social media services on the table in response to several nights of widespread looting and violence in London and other English cities. Police claim that young criminals used Twitter and Blackberry instant messages to coordinate looting sprees in riots.

Prime Minister David Cameron said that the government, spy agencies and the communications industry are looking at whether there should be limits on the use of social media sites such as Twitter and Facebook or services such as BlackBerry Messenger to spread disorder.

The suggestions have met with outrage — with some critics comparing Cameron to the despots ousted during the Arab Spring.

In San Francisco, Sweet said BART board members were told by the agency of its decision during the closed portion of its meeting Thursday afternoon, less than three hours before the protest was scheduled to start.

“It was almost like an afterthought,” Sweet said. “This is a land of free speech, and for us to think we can do that shows we’ve grown well beyond the business of what we’re supposed to be doing, and that’s providing transportation — not censorship.”

But there are nuances to consider, including under what conditions, if any, an agency like BART can act to deny the public access to a form of communication — and essentially decide that a perceived threat to public safety trumps free speech.

BART Deputy Police Chief Benson Fairow said the issue boiled down to the public’s well-being. “It wasn’t a decision made lightly. This wasn’t about free speech. It was about safety,” Fairow told KTVU-TV on Friday. BART spokesman Jim Allison maintained that the cellphone disruptions were legal as the agency owns the property and infrastructure.

He added while they didn’t need the permission of cellphone carriers to temporarily cut service, they notified them as a courtesy.

The decision was made after officials saw details about the protest on an organizer’s website.

He said the agency had extra staff and officers aboard trains during that time for anybody who wanted to report an emergency, as well as courtesy phones on station platforms.

“I think the entire argument that some people think it created an unsafe situation is faulty logic,” Allison said. “BART had operated for 35 years without cellphone service and no one ever suggested back then that a lack of it made it difficult to report emergencies.”

But as in London, BART’s tactic drew immediate comparisons to authoritarianism, including acts by the former president of Egypt to squelch protests demanding an end to his rule.

Authorities there cut Internet and cellphone services in the country for days earlier this year. He left office shortly thereafter.

Aaron Caplan, a professor at Loyola Law School in Los Angeles who specializes in free-speech issues, was equally critical, saying BART clearly violated the rights of demonstrators and other passengers.

Staff writer Matthias Gafni contributed to this report.