A protest organized by the hacker activist collective "Anonymous," intended to draw attention to issues with the Bay Area Rapid Transit (BART) police, turned chaotic and violent last night, resulting in almost half of the demonstrators being arrested.

Protesters did manage to temporarily shut down two BART stations and snarl traffic for a couple hours, but once darkness fell the trouble began.

Area reports place some of the demonstrators breaking off into splinter groups as they marched, running down side streets, tossing trash cans, throwing stink bombs at police and kicking the glass out of bus stop shelters. One person was also arrested on suspicion of "lighting a flammable substance."

The crowd was about 100 people, and authorities initially estimated that 40 were arrested. The protesters were outnumbered by the gathering of police and journalists there to observe the demonstration, according to PC Magazine.

Cellular service on BART platforms stayed on during Monday's actions. A previous last Monday protest largely fizzled when BART officials decided to turn off their platforms' cell receivers, but the tactic backfired in the media and seemed only to galvanize the protesters.

Officials later blamed the protesters for their decision, saying it was made for their safety and pointing to rules which ban protesting on the train platforms.

Anonymous has been urging protests against the BART police over a series of killings many viewed as unwarranted.

A hacker or hackers last week defaced the police department's website and released a list of names, home addresses, email addresses and passwords of the 201 officers of the BART Police Officers Association.

Official Anonymous channels denied that the group was responsible for the hack, suggesting it may have been a supporter or "agent provocateurs."

They've called for another protest next Monday, August 29.