Talks between European unions and representatives from General Motors and Magna are due to start next week over plans to cut some 10,000 jobs at Opel in Europe.
The protesters included employees from Opel's plant in Figueruelas, Zaragoza, as well as local politicians and business and unions representatives, national radio said.
Spanish television carried scenes of protesters with placards carrying messages of support for the workers at the Figueruelas plant.
The closure of part of production at the Figueruelas plant would mean the loss of 1,700 jobs there, as well as a further 800 in auxiliary industries.
The President of the Aragon region 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 told journalists in Madrid.
On Tuesday, the government said it dislikes what it has seen of Magna's plan to buy Opel but would back a European solution which guaranteed the future of the Figueruelas plant.
(Reporting by Judy MacInnes; editing by Marguerita Choy)