The protest began at around midday and is set to continue until Thursday evening.
A group of archaeologists, who are concerned over the future of the profession, entered the famous gallery and blockaded the ticket kiosks, meaning thousands of visitors could enter for free, according to a union group.
"The protesters numbered around 100," said union rep Eric Michon.
Signs were put on the doors of the entrance telling visitors that archaeologists were offering them free visits to the museum.
Des archéologues bloquent les caisses à l'entrée du Louvre: Les manifestants « dénoncent les menaces qui pèsen… http://t.co/k7WLAgp476
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The museum chiefs however have refused numerous times to comment on the protest when approached by AFP, Le Parisien reports.
The protest was prompted by growing concern over the use of private companies to protect France's heritage, which they say has had catastrophic effects over the last 10 years.
"We demand that archaeology is taken out of the commercial sector," said Michon.
They focused on a sector called "preventive archaeology," which helps protect ancient sites, and which was opened up to competition from private companies in 2003.
A spokesman for the culture ministry said it was "astonished" the protesters had allowed the Louvre to "suffer the consequences" of their demonstration and was in talks with the unions over the issue.
The Louvre, one of the most popular attractions in the French capital, is also the most visited museum in the world, receiving 9.3 million visitors in 2014.