Luke Cho, owner of Mildblend Supply Co. in the tony Wicker Park neighborhood, told the Chicago Sun-Times that a number of youths entered the store "in a marching way, like a team" Saturday evening.
"After the 15th or 16th teenager was inside I knew something bad was happening," he said.
Cho locked the door, preventing any one else from entering. He estimated the number of people in the store after he locked the door at "more than 20."
Only Cho and three employees were in the store at the time.
About $3,000 worth of clothing, mainly name brand jeans was taken. The crowd forced aside employees guarding the door, unlocked the door and disappeared into a mass of people attending an outdoor festival before police arrived.
"They basically pushed us aside," Cho said. "We were standing by the door. My staff got cuts and bruises and banged around until [the thieves] unlocked the door and got out."
He has posted security video of the robbery on YouTube and requested that anyone recognizing someone on the video contact Chicago police.