Bay Area Rapid Transit authorities closed San Francisco's Civic Center Station Monday afternoon to control a protest against an earlier cellphone shutdown.

Protesters reportedly stopped a Bay Area Rapid Transit, or BART, train, forcing the authorities to clear the platform for safety concerns.

"Once the platform becomes unsafe, we can't jeopardize the safety of patrons and employees," BART Deputy Police Chief Dan Hartwig told the Los Angeles Times.

BART, which carries about 350,000 commuters daily, shut down wireless networks for passengers at some of underground stations last Thursday for about three hours, in an attempt to thwart planned protests that were expected as retaliation from a hacker group.

The demonstration, which came after a July 3 transit police shooting of an alleged knife-wielding passenger, was being organized through cellphones, the agency believed, based on online discussions regarding the issue.

An estimated 50 protesters gathered Monday afternoon on the Civic Center Station platform chanting "no justice, no peace," according to The Associated Press, and swarmed the BART train in a effort to try preventing its departure.

Law enforcement officials were able to clear the platform and close the station eventually, and BART trains went by without stopping, the news agency said.

The AP reported that an officer told the few remaining protesters that the demonstration was unlawful and warned of arrests if people didn't leave within two minutes.

BART officials told The AP that the decision to cut cell phone service on Thursday, a moved criticized by the American Civil Liberties Union and others, was made out of concern for passengers' safety.

"It's wrong," said ACLU lawyer Michael Risher, whose group was scheduled to meet later Monday with BART's police chief at the agency's headquarters in Oakland. "There were better alternatives to ensure the public's safety."