The hackers said the attack was in retaliation for BART's decision to block cellular telephone service to prevent an anti-police protest, the San Francisco Chronicle reported Monday.
BART said it is considering doing the same thing later Monday in response to Anonymous' call for a 5 p.m. protest at BART's Civic Center Station where police fatally shot a man brandishing a knife last month.
"We're going to take steps to make sure our customers are safe," agency spokesman Jim Allison said. "The interruption of cell phone service was done Thursday to prevent what could have been a dangerous situation. It's one of the tactics we have at our disposal. We may use it; we may not. And I'm not sure we would necessarily let anyone know in advance either way."
Officials said hackers removed information from myBART.org, run by an outside vendor, and published it on another Web site.
The leaked information contained the names and passwords -- and in some cases addresses and telephone numbers -- of more than 2,000 people who use the myBART service, the Chronicle said.
The hackers wasted no time in boasting about their accomplishments, going online to brag about it.
"We are Anonymous, we are your citizens, we are the people, we do not tolerate oppression from any government agency," the hackers said, adding the site had no security and "an 8-year-old with an Internet connection" could have hacked it.
BART is sorry for the loss of information and is examining its Web sites to make them more secure, Allison said.
"We regret the inconvenience and stress that it's created for customers," Allison said. "We're disappointed that they would do this."