Early last month, a BART police officer shot and killed a knife-wielding homeless man at the San Francisco Civic Center station. The incident provoked a series of small protests that drew little attention until Aug. 11, when the transit agency took the unusual step of shutting down cellphone service for several hours as activists prepared for another rally.

With that flip of a switch, BART has come under siege — in cyberspace and underground.

According to officials, BART’s technology personnel have been working round the clock to fend off a disparate group of hackers who penetrated the agency’s Web sites last week and released sensitive information, in retaliation for the shutdown of the cellphone and wireless services.

The Federal Communications Commission is investigating the decision to shut down the services, and the American Civil Liberties Union is considering a lawsuit against BART on the ground of First Amendment violations.

The protests, while still small, appear to have grown in number and potency. Using the Twitter hashtag #MuBARTek, a wry reference to Hosni Mubarak’s efforts to shut down communications before he was toppled as president of Egypt, activists organized a new round of protests that forced BART to shut down four stations during rush hour last Monday.