(NewsNation Now) —  While retailers are thriving on holiday sales, retail theft is plaguing the industry. A wave of robberies were brazenly carried out recently in high-end stores from Chicago to Northern California by flash mobs, swarms of thieves who pull up together, work quickly and leave, all in plain view.

This weekend, two luxury stores were ransacked in the San Francisco Bay area, including a Nordstrom store targeted by a crowd of 80 people. Three are now in custody.

“They were running down the street,” a witness said. “I probably saw like 50 to 80 people in ski masks, crowbars, a bunch of weapons. They were looting the Nordstrom right here.”

A Louis Vuitton store in downtown San Francisco’s Union Square also fell victim.

San Francisco Mayor London Breed said police presence will be increased and called it a crime against the whole city.

“When people vandalize and commit those level of crimes in San Francisco we not only lose those businesses, we lose those jobs, we lose that tax revenue that helps to support our economy, that helps to support many of the social service programs,” Breed said.

A similar incident happened Friday in the Chicago area when a large group of people robbed a Neiman Marcus department store on Michigan Avenue, one of the city’s busiest thoroughfares.

“About 18 offenders entered a store and they took approx. 20 handbags with an estimated value of $50,000 and fled the scene,” said Brendan Deenihan, chief of detectives with the Chicago Police Department.

Chicago police say that some of the bags have been recovered while the investigation continues.

About 20 miles outside the city in suburban Oak Brook, 14 suspects made off with nearly $120,000 worth of merchandise from a Louis Vuitton store last week. One of three getaway cars was found over the weekend.