A gay-rights demonstration was cut short Wednesday in Casa Grande when a police officer threatened participants with arrest for flying a pride flag inside city limits, organizers allege.

The Casa Grande Police Department is investigating the incident.

Police say the officer was called to the area on a report that the group's flag, which measured 8 feet by 5 feet, was obstructing the view of traffic near an intersection.

Group members say the officer crossed the line.

"I was upset more than anything and confused as to what was the problem," said Christopher Hall, protest organizer. "This was wrong. They've actually violated, in my opinion, First Amendment rights."

Hall, a 19-year-old student at Central Arizona College, is the president and founder of Central Arizona Rainbow Equality, a month-old organization that serves members of the lesbian, gay, bisexual and transgender community in Pinal County. He said that Casa Grande police have been good partners in the past during pride events.

He and four CARE members were demonstrating in protest of taxation without representation in a dirt field on the northeast corner of Trekell Road and Cottonwood Lane. Hall said they stood at least 5 feet from the sidewalk, as instructed by a city official when Hall checked to make sure the event would be in compliance.

Someone called 911 at 6:02 p.m.

Police on Thursday said they knew little about what happened when Officer Brian Walsh, a four-year veteran, responded to the scene. Patrol Commander Scott Sjerven said Walsh's notes following the incident showed the group was "made to leave (and) warned to not obstruct the view of traffic by flying their flag."

Hall said he and the flag's owner complied by giving the officer their identification and asked where they might move their demonstration. That's when they were told they couldn't fly the flag "anywhere" or risk arrest, Hall said.

Alessandra Soler Meetze, executive director of the American Civil Liberties Union of Arizona, said these incidents can be avoided with adequate training of officers who most often encounter free-speech issues. Walsh had an obligation to ensure public safety if the protesters were blocking traffic, but she said it appeared he overstepped his bounds.

"That kind of police hostility chills free speech," Soler Meetze said. "You simply can't suspend people's free-speech rights because you disagree with the message."

Hall's group reported the incident to the ACLU on Thursday. An ACLU legal adviser plans to meet with them.

A statement issued Thursday afternoon by Casa Grande police Chief Robert Huddleston said the department is working to determine whether Walsh acted inappropriately and, if so, will take steps to correct issues. Huddleston said he will meet with Hall and other CARE members Monday.

"The Casa Grande Police Department will continue to work with this and any other citizen group to respect their right to assemble and demonstrate in a safe manner," Huddleston said. "We apologize for any inconvenience or misunderstanding, and look forward to the completion of the investigation."