Staff at Opel plants in Germany and its sister British brand, Vauxhall, are expected to hold a demonstration against the threat of job cuts.
"We want to expose our demands clearly once again," Rainer Einenkel, the head of the works council at GM's plant in Bochum, western Germany, told AFP Tuesday.
Einenkel said he would attend a meeting of Opel's supervisory board on Friday where management would present an "Opel rescue plan" at the meeting at the request of the German government.
Under German law, labour representatives are granted 10 seats out of 20 on a company's supervisory board.
Echoing Einenkel's call to protest, the European Metalworkers Federation (EMF) said in a statement Tuesday that it would oppose any forced plant closures or job cuts.
Leaflets handed out at Opel's main plant in the western Germany city of Rüsselsheim demanded that there should be "No plant closures and no layoffs" and called for workers to protest Thursday.
Germany's Foreign Minister Frank-Walter Steinmeier, the Social Democrat candidate in September's general elections, is expected to attend the rally.
Berlin told Opel to come up with a viable business plan if it wants to benefit from state aid, with unions saying the company needs at least €3.3 billion ($4.2 billion) to survive.
The Financial Times Deutschland reported Tuesday that Berlin may be willing to pump money into Opel, without citing sources.
Another option is acquiring a direct stake in the ailing automaker, although the idea is opposed by some within the country's ruling coalition.
In Sweden, workers in Saab's Trollhaettan plant were also urged to rally after the company filed for bankruptcy protection last Friday.
Swedish authorities have flatly refused to provide Saab with direct aid, but have said they would only step in as a guarantor on loans from the European Investment Bank if a new buyer could be found.
Scarce consumer credit has seen auto sales plummet worldwide, with Saab badly hit by its ageing product line and the strength of the Swedish krona to the US dollar - its largest export market.
GM employs 55,000 workers in Europe, primarily in Britain, Germany, Spain and Sweden. The company wants to restructure its European organisation as it looks to stem huge losses.