The international group of hackers known as Anonymous began a cyber attack on the San Francisco subway's online presence Sunday, in retaliation for the agency's bid to shut down cellphone service to limit a protest over a fatal shooting.

As of late Sunday the main website for the Bay Area Rapid Transit (BART) was still functioning, but the San Francisco Chronicle and other media reported the hackers had defaced a related website -- mybart.org -- and leaked user's personal information.

BART made a "conscious decision of ordering various cell phone companies to terminate services for the downtown area inhibiting those in the area from using cell phones -- even in the case of an emergency," said the group in a statement posted online.

"Anonymous will attempt to show those engaging in the censorship what it feels like to be silenced," said the group.

The subway service said earlier Sunday it was "disappointed" to announce their website "may be subject to an online attack today, Sunday August 14, between noon and 6:00 pm (1900 GMT to 0100 GMT Monday)."

The action, said BART, would "directly affect those customers who depend upon our site, as well as the developers who use BART's open data services to serve BART customers."

"We're not done yet folks... we're just getting warmed up," announced the group on one of their affiliated Twitter accounts.

The original protest on Thursday was to protest the shooting death in July of a knife-wielding homeless man on the BART system at the hands of police, which connects some two million people per month with the San Francisco area with the outer suburbs and airport.

The group called for a peaceful demonstration on Monday against BART, urging protesters to wear red shirts and masks.

"Hacktivists" with Anonymous previously hacked the financial organizations Visa and PayPal, among others, after they cut supply funds from the activist whistleblower site Wikileaks.