Around 12:30 p.m. Wednesday, the Salle des États had only a few dozen visitors, cellphones aloft, taking selfies with the Mona Lisa. Usually, hundreds of people wait in line to catch a glimpse of the masterpiece, but uncertainty about whether the museum would open on Wednesday kept many away.

One guard, Quentin Osle-Rudler, stood six feet from the painting, keeping watch. He said he was unsure about the new directive not to move around the room. “People are going to wait around in front of the painting,” he added. “It will be messy.”

He said he was concerned that the Salle des États would become too full as the crowds returned to the museum, increasing the risk of transmission for the virus.

Mr. Galani, the union representative, said further measures would be needed. “The problem is that once the room is full of people, I don’t see how agents could not be exposed to the virus,” he said. “At some point we will probably have to regulate the number of visitors in the room.”

Over recent days, the Louvre has been turned into a test case for how large museums should respond to the coronavirus outbreak. France has over 200 confirmed cases, with several deaths.

On Saturday, the French government banned all indoor events of more than 5,000 people, including arena concerts. The popular rapper and singer Maître Gims was one of several artists who postponed a planned tour. The French culture minister, Franck Riester, said the ban did not apply to the Louvre because visitors were dispersed throughout many rooms of the museum. But the Louvre’s employees were not convinced, and voted to close on Sunday.