Police said tourists have reported having their pockets picked while they were distracted by people wearing white sheets with their faces painted white, The Local.se reported Tuesday.
"Thieves are always finding new methods. Often, the thievery is carried out using some kind of distraction. These people in particular are very blunt and pushy, which perplexes people," Stockholm police spokesman Gunnar Thun told the Dagens Nyheter newspaper.
"Many tourists think that Stockholm is a safe city. This is true, perhaps, when it comes to violent crimes, but not when it comes to personal property crimes. Things are stolen very often here," Thun said.
He warned tourists to be on the lookout for the "ghosts."
"I would keep them at an arm's length," he said.