The group of hackers known as "Anonymous" claimed responsibility for an online attack Sunday against the San Francisco transit system, and threatened further protests during Monday evening's rush hour.

The hackivists took down the Bay Area Rapid Transit System's Web site over the week following the system's cutting off of cellphone service in a bid to thwart an Aug. 11 protest at the subway station that was being coordinated with mobile devices.  The original protest centered on how BART has handled several shootings involving transit police officers.

The hacker group gained access to the system's internal network which contained personal data on thousands of customers, including e-mail and home addresses as well as phone numbers. Officials have said no sensitive financial information was gleaned.

Anonymous reportedly said that "probably the only information that will be abused from this database is that of BART employees."

Anonymous said it plans a new demonstration starting at 5 p.m. PST Monday at the transit system's Civic Center station. BART has advised commuters that some stations may close temporarily on short notice for safety reasons.

BART's Web site appeared to be functioning normally on Monday. A link off its site over the weekend led to a page displaying the hackers' logo of a smirking Guy Fawkes' mask above two crossed swords on a black background.

A video posted online on the afternoon of the attack stated that BART had angered the group Anonymous. A computer-generated voice from the group said that by cutting cell service, "You have not only threatened your citizens' safety, you have also performed an act of censorship."

BART said in a statement that it is taking steps to remedy the situation. The hackivists have also claimed responsibility for taking down other sites including PayPal, Visa and the Church of Scientology.