Screen capture by Eric Mack/CNET

Hacktivist group Anonymous says it will take the Web site of the Bay Area Rapid Transit (BART) system off line later today for six hours while also inundating BART fax lines and e-mail accounts. A press release published online detailing the group's plans says the actions are in retaliation for BART's unilateral shutdown of cell phone service Thursday night at some BART stations to prevent another planned protest.

The Thursday demonstration had been planned to protest the fatal shooting of a man by a BART police officer last month. BART officials said they took cell phone service offline in an attempt to disrupt the planned protest, which was to be coordinated via mobile devices, because they worried the demonstration could "lead to platform overcrowding and unsafe conditions for BART customers, employees and demonstrators." An earlier protest last month had disrupted BART service.

Chatter about #OpBART cropped up Friday on Twitter, but few details could be found until plans for a peaceful protest Monday evening outside a central San Francisco BART station began circulating. That was followed with this posting late Saturday of the planned attack on BART's e-mail, faxes, and Web site:

Anonymous will take the following actions over the next 48 hours.

1) We have begun at once a massive Black Fax and E-Mail Bomb action, where we will fill every inbox and fax machine at BART with thousands of copies of our message that this outage was unacceptable

2) Tomorrow, Sunday - August 14, 2011 at High Noon Pacific Time we, Anonymous - will remove from the internet the web site of BART located at www.bart.gov for exactly six hours. That's twice as long as they shut off the cell phones for.

3) On Monday - August 15, 2011 at 5:00 PM Pacific Time there will be a physical protest at the Civic Center Bart Station. Expect us !

The following video, posted Saturday to YouTube, encourages San Francisco residents to wear a red shirts "in remembrance of those who have been battered by the BART police" and masks to the Monday protest and to bring cameras to "record any further abuse by police."

BART spokesman Jim Allison tells CNET that they are aware of both the planned cyberattacks and Monday's planned protest. He would not elaborate on any plans to deal with either, but said that BART will "continue trying to provide information to our customers [via the BART Web site.]"

Allison says he has not yet heard or seen any evidence of the e-mail and fax "bomb" campaign that the Anonymous release claims is already underway.

As Sunday commuting was just getting underway, at least one sympathetic Twitter account was already reporting support for Anonymous' cause:

"Wow...walking to the @SFBART and I'm starting to see people with masks on. "