The ACLU is basing its concerns on a tape that shows deputies and other officers dressed in riot gear breaking up a vigil for Alfred Olango, who was shot and killed after a confrontation with El Cajon police officers last Tuesday. More than a dozen people were arrested or detained after refusing to leave, police said.
"Based on what we do know, this looks like an unconstitutional arrest," said David Loy, with the San Diego, Imperial County office of the ACLU.
Loy said the protesters had a right to gather considering they were doing it in a peaceful manner and were on private property.
The ACLU is still waiting to hear back from the El Cajon Police Department and other law enforcement agencies involved.
"Please tell us if there's something we don't know. We're very troubled and concerned that this was an unconstitutional attempt to suppress a lawful and peaceful protest," said Loy.
Over the weekend, El Cajon police Lt. Rob Ransweiler released a statement saying the police decided to disperse the demonstrators after a fight broke out between a few protesters. Someone reported that one of the protesters was leaving to get a gun.
"Sensing this shift in the demeanor of the crowd and out of concern for community safety, officers declared an unlawful assembly and ordered the group to disperse," Ransweiler said in the statement. "The majority of the group left at this time, however some remained."
Police arrested 12 people who refused the order to leave, Ransweiler said.