As hundreds walked through Baltimore on Thursday to protest a police shooting in Missouri, uniformed officers were recording their every move.

The decision by the Baltimore Police Department to record the march raised questions about how the tapes would be used and why authorities had chosen to record the event. Participants said they saw as many as a dozen officers shadowing the crowds and filming them, almost like a documentary crew, as the protesters walked to the Inner Harbor or spoke with officers.

"They police didn't violate anyone's rights, but what does it mean that these pictures are being taken?" said activist C.D. Witherspoon. "It had nothing to do with security, because there were tons and tons of officers there."

The Police Department declined to discuss the issue but put out a statement saying filming such events is now considered a "best practice" for law enforcement agencies, and said the tapes would be used for training purposes. Nationally, the Police Executive Research Forum, an influential think tank, has been advising departments to film events to protect officers from false claims and to review officers' actions.

David Rocah, a staff attorney for the American Civil Liberties Union of Maryland, said he has concerns about the department's rationale.

"When police are recording demonstrators, they're recording people exercising their First Amendment rights, [which] can itself be intimidating and could lead to police creating a database of who is protesting what. And if it was used in that way, it would be wrong, unconstitutional and deeply disturbing," Rocah said.

The Police Department said making a recording is "a way to document all aspects of an event."

"The recordings document officers' actions, commands given to units, and interactions between police and members of the public," police said in a statement Friday. "Recordings of the public can be used for training, documenting, and ensuring the highest professional standards of officers involved in protecting large-scale gatherings and events."

"Our mission was to ensure that our residents had an opportunity to express their first amendment rights," Police Commission Anthony W. Batts added in an e-mail Saturday.

Lt. John Kowalczyk, the department's chief spokesman, said officials were drafting procedures to govern the practice. "You will see us continuing to film at large scale events such as New Year's Eve," he said in an email.

A 2011 report by the Police Executive Research Forum recommends that police officials not "be afraid to record video of major events, including your own officers."

"Having your own video recordings is critical to reviewing officers' actions," the report says, adding that citizens and the news media will be filming and "may edit recordings to create false impressions or show incidents out of context."

Philadelphia Police Commissioner Charles Ramsey says in that report that when he was chief of police in Washington, police didn't want to film for fear of capturing "something that we didn't want to see." But he said having an independent account of anything that transpired was more important.

The New York Civil Liberties Union contends police can't legally record protesters unless "it reasonably appears that unlawful conduct is about to occur, is occurring or has occurred." It says it is investigating how New York police use video recordings.

Seattle police recommend video units "for events where intelligence indicates that civil disobedience or crowd violence will occur" and has guidelines for how recordings are handled, according to the department's website. In Washington, police have rules outlining how to log and store recordings of any "mass demonstration or protest."

The ACLU of Maryland sued the Maryland State Police in 2008 to get records about covert infiltration and documentation of peace activists and anti-death penalty groups, which showed that police kept logs and entered some names into a database of people thought to be drug traffickers or terrorists.

The civil rights group has advocated for the rights of citizens to record police interactions, and for the use of body cameras on officers, though it has some concerns about the practice.

Rocah questioned why city police filmed the protest as opposed to other types of incidents handled by officers.

"The view that protesting is itself somehow an inherently disorderly or problematic activity is troubling and the wrong attitude for police to take," Rocah said.

But he saw few problems with police recording the crowds for the purpose of having evidence if a disturbance were to take place, and he acknowledged that rolling cameras were likely to improve the behavior of everyone involved.