Spanish workers have taken to the streets of Zaragoza to protest at the mooted sale of Opel and Vauxhall to Magna.
If the deal goes ahead, Magna plans to cut around 10,000 jobs across Europe. The Opel plant in Figueruelas, Zaragoza, where 1700 people are employed, is believed to be under particular threat.
The president of the Aragon region of Spain said his government would not accept any decision to close the plant and relocate production.
"We will fight tooth and nail to defend every job at the plant," Marcelino Iglesias said.
Talks between Magna and workers' unions throughout Europe are set to begin this week.