Daimler AG, which owns Mercedes-Benz, will likely decide on the move on Tuesday, and the union fears that it could mean the end of more than 3,000 jobs at the plant in Baden-Württemberg.
“The production of the C-Class is of central importance for the workers at Sindelfingen,” works' council head Erich Klemm said on Monday.
At the protest on Tuesday he threatened to “turn the lights off” at the factory if the factory change moves forward.
Several thousand workers were on hand at the country's largest car factory for the protest, including many from carmaker Porsche, auto supplier Bosch, and other businesses that would be affected by the move.
A factory in Tuscaloosa, Alabama is said to be in the Daimler's sights for further C-Class production due to a favourable exchange rate and better sales market. The company currently build four out of five of its vehicles in Western Europe, but sells only 60 percent in the region.
Some 20,000 people work at the Sindelfingen factory, 4,500 of whom specialise in C-Class production.