The U.S. Federal Communications Commission should take swift action to rule against the Bay Area Rapid Transit (BART) District's recent decision to shut down mobile phone service during a planned protest, several digital rights groups said Monday.

BART's decision to cut mobile service in its subway system on Aug. 11, as a way to disrupt planned protests, violates the U.S. Communications Act and the First Amendment to the U.S. Constitution guaranteeing free speech, said the groups, including Public Knowledge, the Center for Democracy and Technology and the Electronic Frontier Foundation.

BART's actions also raise concerns about public safety, because mobile phone users on the BART system were not able to dial police or other emergency responders during the three-hour outage, said Sherwin Siy, deputy legal director at Public Knowledge.

"Unilateral action by law enforcement, however well intentioned, risks depriving the public of vital emergency communications at the worst possible moment," the groups said in a petition filed with the FCC. "Because any impairment of [mobile service] impacts both critical issues of public safety and important principles of free expression, the Commission must act swiftly to clarify that local authorities may not turn off wireless networks before other local jurisdictions seek to replicate the actions of BART."

The emergency petition asks the FCC to rule immediately so that other government agencies don't try to copy BART's tactics. BART appears ready to shut down mobile phone service again for similar reasons, the groups said.

"The petition isn't really about punishing BART so much as making sure that this sort of problem won't arise in the future," Siy said. "We hope that the FCC will clarify for other local authorities around the country that shutting off cell service isn't just a bad idea, it's also against the law."

BART's actions are a violation of established U.S. law, "plain and simple," Siy said. The Communications Act says telephone carriers and their agents cannot "discontinue, reduce, or impair service to a community, or part of a community" without FCC approval.

In addition, California's Second District Court of Appeal, in 1942 case People v. Brophy, ruled that no state official has the authority to suspend phone service on the "mere assertion that illegal activity might take place," the petition said.

BART has defended its actions, saying it cut service to ensure safety. "Organizers planning to disrupt BART service on August 11, 2011 stated they would use mobile devices to coordinate their disruptive activities and communicate about the location and number of BART Police," BART said in a statement posted to its website. "A civil disturbance during commute times at busy downtown San Francisco stations could lead to platform overcrowding and unsafe conditions for BART customers, employees and demonstrators."

Protestors had planned to chain themselves onto BART subway cars, potentially trapping more than 8,000 passengers in a 3.4-mile tunnel at the bottom of the San Francisco Bay, said Bob Franklin, president of BART's board of directors. An emergency situation in a trapped train would have been difficult to respond to, he said.

"The intent was to protect passenger safety, not to stop a protest," he added.

Asked about the loss of emergency dialing service, Franklin said BART had police officers and other workers stationed on trains and platforms during the mobile phone outage.

"Every few feet, there was either a police officer or someone with a radio," he said. "If you had an emergency, you turned to your right or your left and said, 'I have an emergency.'"

BART's board is considering a new policy that would shut down mobile phone service only in extreme circumstances, Franklin added. The protests may not qualify as an extreme situation in the future, but BART police will need some flexibility to deal with issues on a case-by-case basis, he said.

The FCC is looking into BART's decision to cut off mobile service, according to an agency spokesman.

The Aug. 11 protest didn't happen. It was planned in response to the July 3 fatal shooting of a knife-wielding BART passenger by BART police.

Grant Gross covers technology and telecom policy in the U.S. government for The IDG News Service. Follow Grant on Twitter at GrantGross. Grant's e-mail address is grant_gross@idg.com.