SAN FRANCISCO (Reuters) - The hacker group Anonymous is taking on San Francisco's subway system after the Bay Area Rapid Transit turned off cell phone service in stations last week to foil a different protest. The transit service last Thursday shut down cell phone networks in some stations to stop a protest over a police shooting. In a city known for civil rights protests and technology, that tactic has landed the commuter train system in a new controversy. "We've invited these protesters now," said Lynette Sweet, a BART director.