Police officers in San Francisco arrested 45 people Monday evening during a protest at commuter train stations organized largely online by a group of computer hackers called Anonymous. The demonstration was the latest since transit officers shot a 45-year-old man to death on July 3. Demonstrations involving more than 100 protesters began around 5 p.m., closing two stations during peak commuter hours and snarling traffic downtown for more than three hours. During an Aug. 11 protest, transit officials shut off cellphone service in transit stations in an effort to thwart protesters. The move angered online activists, who retaliated by hacking the Bay Area Transit Authority’s Web site and releasing the personal information of users.