Anonymous is anyone, and therefore it's everyone.

It's a cause, an idea, a network, a rallying point, its supporters say.

But it's a movement that has no structure, no leader, no rules. And yet this is the group - and many of its supporters don't even consider it a group - that gave BART a major headache last Monday when a protest organized online forced the agency to close its four downtown San Francisco stations during evening rush hour, stranding thousands of commuters.

Today, Anonymous is promising more of the same, planning a protest for 5 p.m. at Civic Center Station - the same time and place as last week's demonstration.

As part of the Anonymous campaign known as OpBART, the group promises to keep up pressure until BART officials admit they were wrong to cut off cell-phone service in underground stations to head off an earlier planned protest, apologize to the public and fire the transit agency's chief spokesman, who defended the cutoff and has been outspoken in his denunciation of "cyber-thugs."

Sometimes called a proponent of "hacktivism," Anonymous generally targets institutions its supporters consider to be suppressing free speech and Internet freedom. Earlier this year, some blamed its supporters for a sophisticated attack on Sony's PlayStation Network over the company's pursuit of PS3 hackers, an attack that forced the system offline.

In December, Anonymous initiated an online attack against PayPal after the company stopped processing donations to WikiLeaks. When the FBI arrested 14 Anonymous members last month in connection with the action, the group asked supporters to boycott PayPal.

Offline

In 2008, Anonymous went offline, organizing demonstrations at Scientology centers around the world. But several supporters in Anonymous chat rooms - the forum where its plans are hatched and organized - celebrated the Aug. 15 BART protest in San Francisco as one of the movement's first strikes outside the confines of the online world.

"We're not just cyber anymore," said Peter Fein, 33, a computer programmer from Washington state and one of the few Anonymous supporters willing to reveal his identity. "This is as good as a protest gets. There were no arrests, no extensive injuries, no damages. This is how it should be."

While Anonymous' faceless, online organizing technique makes it a difficult target for law enforcement to pin down, it also leaves the movement vulnerable to internal disagreements. When the Anonymous Internet Relay Chat network was attacked and rendered unusable Thursday evening, OpBART organizers initially said they feared the perpetrator was an Anonymous supporter opposed to their protests. They later found out it was someone with a personal grudge against some of his Anonymous friends.

"Anonymous is not unanimous," said one of OpBART's organizers, who is known by the pseudonym Po and identifies himself only as a computer programmer living in the United States.

Some of the Anonymous actions taken in the name of OpBART have run into criticism even from those who otherwise back the campaign. Po, who was interviewed online, said he opposed the Anonymous-backed hacking last weekend of the marketing website myBART.org, which resulted in the online posting of more than 2,000 customers' personal information.

But because Anonymous does not control individual actions, there is nothing organizers can do when an individual goes rogue.

"It's a matter of timing and 'force of will,' " said an OpBART organizer known as AlbaandOmegle. "The actions we take (as a whole) have to drown out the bumps."

Another problem for the Anonymous movement is that because anyone can claim membership, any hacking can be attributed to Anonymous.

When a French girl known only as Lamaline_5mg hacked the BART police association's website last week and released the personal information of 102 officers and other employees, many assumed the action was the work of Anonymous. The girl turned out to be acting on her own.

In addition to being angered by BART's cell-phone cutoff Aug. 11 - an action the agency did not repeat during last week's actions - OpBART supporters said that they were seeking to "amplify" earlier protests over the July 3 fatal shooting of a knife-wielding transient by a BART police officer. Among Anonymous' demands is that BART retrain its officers to minimize the use of deadly force.

Changes ignored

Linton Johnson, the BART spokesman whom Anonymous wants fired, said the movement is overlooking police reforms that BART has instituted since the New Year's Day 2009 incident in which then-Officer Johannes Mehserle fatally shot the unarmed Oscar Grant. Johnson said BART has tripled police training and created two independent bodies to keep police accountable.

"It's ironic that these protesters have no idea about the sacrifices that people who live and care about this community made after Oscar Grant, and all the sweat and tears they poured in to create citizen oversight," Johnson said. "Use the process that your predecessors put in place and let the people get to their jobs and loved ones."

As for Anonymous' call for his firing, Johnson said, "Regardless of what they think of me, let's have a discussion outside the fare gates and let the customers get from point A to point B."

Among those planning to protest at the Civic Center Station this evening is Fein, who is making the trip from Washington state.

"If our only actions are online, then we're shouting in vain," Fein said. "It's the year the Internet crossed back over into making a difference in real life."