Hackers have targeted the website of a San Francisco transport company that turned off mobile masts to prevent protests outside its stations.
Public anger towards the Bay Area Rapid Transport (BART) had been fermenting since July when transit police shot dead a man who was carrying a knife.
BART disconnected its transmitters in a bid to quell planned demonstrations.
Hacker group Anonymous retaliated by attacking myBART.org, and releasing users' personal information.
The hackers posted details of over 2,000 myBART users online.
In a typically-worded statement, the hacking group said: "We are Anonymous, we are your citizens, we are the people, we do not tolerate oppression from any government agency.
"BART has proved multiple times that they have no problem exploiting and abusing the people."
BART's main website, BART.gov, was left unaffected.
However, myBART.org - which carries offers and extra services for passengers - was defaced and, as of Monday, remained offline.'Disruptive activities'
An action group has called for the disbanding of the BART police force following the July 3 shooting of a homeless man Charles Hill and also the 2009 killing of another man, Oscar Grant.
Protesters have already been successful in rendering some stations on the network unusable by staging mass sit-ins.
BART took the decision, on 11 August, to shut off mobile phone reception at some of its sites.
In a statement, the transport company said: "BART temporarily interrupted service at select BART stations as one of many tactics to ensure the safety of everyone on the platform."
It said protesters had stated that they: "would use mobile devices to coordinate their disruptive activities and communicate about the location and number of BART Police".
BART added: "Disturbance during commute times at busy downtown San Francisco stations could lead to platform overcrowding and unsafe conditions for BART customers, employees and demonstrators."UK troubles
The move was widely criticised as being heavy-handed, with some San Francisco residents suggesting that blocking phone use was a violation of free speech.
BART's decision has parallels with recent troubles in England where days of riots were sparked when 29-year-old Mark Duggan was shot dead by police in Tottenham, north London.
Prime Minister David Cameron suggested in Parliament that access to social networks could be blocked in times of civil unrest.
His comments came after it emerged some of the looting and violence was co-ordinated by people using services such as Blackberry messenger.
His statement was widely condemned by rights campaigners who said that such measures could be an attack on civil liberties.
On Monday, BART warned of further disruption to the service, and did not rule out that similar blocking-measures could be used.'Completely unjustified'
However, reaction to Anonymous' intrusions has been less than supportive.
Laura Eichman, whose phone number was published along with the stolen personal data, said: "I think what they [the hackers] did was illegal and wrong.
"I work in IT myself, and I think that this was not ethical hacking. I think this was completely unjustified."
Anonymous has argued that anger should be directed at BART's "unsecure" data protection methods, rather than the group.