(CNN) -- Members of a well-known hacking group -- according to a statement and Twitter messages -- took credit Sunday for an online attack targeting San Francisco's embattled transit system.
Anonymous -- in a news release attributed to the group, and backed up by related Twitter pages -- said it would take down the website of the Bay Area Rapid Transit System, known as BART, between noon and 6 p.m. PT Sunday. This is in response to the system's decision to cut off cellphone signals at "select" subway stations in response to a planned protest last week.
"By (cutting cell service), you have not only threatened your citizens' safety, you have also performed an act of censorship," a seemingly computer-generated voice -- speaking over dramatic music and images -- said in a video posted online Sunday afternoon. "By doing this, you have angered Anonymous."
On Sunday afternoon, a link off BART's website to myBART.org apparently had been hacked. It showed a page featuring, among other items, the Anonymous logo -- a smirking mask above two crossed swords, all on a black background.
In addition, Twitter traffic related to Anonymous boasted that hackers had been able to get into BART's internal network. Several related items and documents were posted, including one claiming to be "the User Info Database of MyBart.gov." This had e-mails and, in some cases, phone numbers of hundreds of people.
"We apologize to any citizen that has his information published, but you should go to BART and ask them why your information wasn't secure with them," the posted item said. "Also do not worry, probably the only information that will be abused from this database is that of BART employees."
A call to a BART communication officer was not immediately returned.
In a statement on its website released earlier in the day, BART acknowledged that it was aware of the threatened online attack -- and was taking steps in response.
"We're doing what we can to defend against any attack on the BART website," the system said. "BART's website infrastructure is wholly separate from any computer network involved in the operation of BART service."
In recent years, the hacking collective calling itself Anonymous has targeted several organizations to make political statements.
Last week, for instance, Anonymous promised an "operation" targeting Facebook. And it took credit for hacking the site of the Syrian Defense Ministry, replacing its content with an anti-government message and the Anonymous logo.
Anonymous also has boasted about taking down numerous other prominent websites, including those of PayPal, MasterCard, Visa and the Church of Scientology.
The action in San Francisco is the latest challenge for the transit system, which has faced criticism after several shootings involving its police officers -- the latest coming last month and resulting in the death of 45-year-old Charles Hill.
Demonstrators had planned a rally last Thursday to bring attention to the issue. But BART contended that such a protest during rush hour endangered the safety of commuters and employees.
In response, the system said in a statement that "it temporarily interrupted service at select BART stations as one of many tactics to ensure the safety of everyone on the platform." The move was made on the same day that British Prime Minister David Cameron proposed a crackdown on social media to quell riots.
The statement from Anonymous also touted a new peaceful demonstration that it said would occur at 5 p.m. Monday at the transit system's Civic Center station.
"We sincerely hope that this series of actions will serve as a warning to BART and every public organization in the USA to NOT engage in this sort of dangerous and human rights violating behavior," the statement said.
In the video statement linked to Anonymous and posted Sunday, a voice urges demonstrators to wear red shirts "in remembrance of those who have been battered by the BART police" and bring a camera to record the events.
"We will show the world and BART that we will not stand for these types of actions," the voice said.
BART issued another alert Sunday advising riders about the possible Monday protest.
"As always, BART's primary mission is to ensure your safety and keep trains on-time," said a statement on the system's website. "BART may need to close some stations temporarily or make other service adjustments on short notice."
CNN's Divina Mims and Greg Botelho contributed to this report.