The hacker group "Anonymous" took the action in response to a BART decision last week to shut off wireless service in hopes of stifling an anti-police protest.
Anonymous posted contact information for more than 2,000 BART customers. BART has since shut down its public-relations website, the target of the attack.
Cellphone service was operating Monday night as an estimated 50 protesters gathered on the Civic Center Station platform chanting "no justice, no peace" shortly after 5 p.m. Thirty minutes later, police in riot gear and wielding batons closed the station and cleared the platform after protesters briefly delayed an east-bound train from departing.
From Civic Center, the protesters were joined by more demonstrators and marched down San Francisco's Market Street and attempted to enter to more stations
The shutdown of wireless towers in stations near the protest last Thursday night helped raise questions about the role that social networks are playing in helping people, from Egypt to London, organize online. In the U.S., with its history of free speech, critics are saying BART's move was unconstitutional.
BART officials have said their primary concern was to ensure that passengers are safe. BART spokesman Linton Johnson declined to answer questions about why the agency decided to keep the wireless system operable Monday.
The Associated Press contributed to this report.