The American Civil Liberties Union filed a lawsuit on Thursday to stop the Baltimore Police Department from testing one of the most expansive surveillance regimes in any American city, an aerial photography system capable of tracking the outdoor movement of every one of its 600,000 residents.

Last week the Baltimore Board of Estimates approved a police contract with Persistent Surveillance Systems LLC to let the company and police fly three airplanes outfitted with high-resolution cameras over the city. According to the contract, the imaging systems can photograph up to 32 square miles every second, allowing for the slow-motion reconstruction of virtually all outdoor movement.

Baltimore police representatives have previously stated that the intent of the Aerial Investigation Research program is for the planes to fly simultaneously, allowing them to record imaging for 90 percent of the city.

In their complaint, lawyers for the ACLU call the system a “society-changing threat to individual privacy and to free association” and argue that it violates constitutional rights to privacy and free association.

“The data collected through the AIR program will amount to a comprehensive record of the movements of Plaintiffs and nearly everyone in Baltimore — facilitating an unprecedented police power to engage in retrospective location-tracking,” the complaint says. “The AIR program would put into place the most wide-reaching surveillance dragnet ever employed in an American city, giving [Baltimore police] a virtual, visual time machine whose grasp no person can escape.”

The lawsuit was filed on behalf of a community organization called Leaders for a Beautiful Struggle, which has advocated for racial justice and police reform in the city, as well as by two other Baltimore activists and community organizers. The ACLU argues that constant aerial surveillance would “undermine the ability of LBS to carry out political activities crucial to its mission.”

The Baltimore Police Department did not immediately respond to a request for comment.