The staff is protesting a growing problem with pick-pocket thieves. That forced visitors to be turned away.
The museum says the pickpocket problem is rising, despite tighter security measures within the past year.
Many of the thieves are said to be gangs of children, who get into the museum for free, surround visitors or employees, and clean out their pockets.
The union says the culprits spit on, insult, or even attack guards at the museum.