In a controversial move that has riled up free speech advocates, San Francisco's Bay Area Rapid Transit (BART) subway system said it cut off cellphone signals at “select” stations in response to a planned protest this week.

“BART temporarily interrupted service at select BART stations as one of many tactics to ensure the safety of everyone on the platform,” the transit agency said in a statement on its website Friday.

BART said it took the actions because protesters said they “would use mobile devices to coordinate their disruptive activities and communicate about the location and number of BART Police.”

Demonstrators had planned a rally to bring attention to a number of transit police officer shootings, the latest one resulting in the death of 45-year-old Charles Hill, who was shot last month after a confrontation with officers.

The transit agency said protests during rush hour endangered the safety of commuters and employees.

“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,” the agency said.

The incident happened Thursday, the same day that British Prime Minister David Cameron proposed a crackdown on social media to quell riots.

"Everyone watching these horrific actions will be struck by how they were organized via social media," Cameron said Thursday during an address to Parliament. "Free flow of information can be used for good. But it can also be used for ill. And when people are using social media for violence, we need to stop them."

Protesters in San Francisco have used websites and social media to organize demonstrations, including a rally last month that shut down a subway stop.

On Saturday, a BART director said the cellphone shutdown was not authorized by higher-ups and was under investigation, according to the Bay Citizen newspaper. “This is a transit agency, and our job is not to censor people,” BART official Lynette Sweet was quoted as saying.

The ACLU also denounced the cellphone shutdown, likening it to strong-arm tactics used by other governments.

“Shutting down access to mobile phones is the wrong response to political protests, whether it’s halfway around the world or right here at home,” the ACLU of Northern California said on its website.

The petition site, Care2.com, started an online petition titled “BART: Stay Out of Our Cell Phone Service!” On Saturday evening the site had more than doubled its signature goal of 1,000.

Also the hacktivist group Anonymous said it would be targeting BART on Monday to retaliate, several news sites reported Saturday.