Thousands of workers from Italy's auto group Fiat marched through the streets of Turin on Saturday shouting for job guarantees.
Fiat is planning the acquisition of the General Motors German subsidiary, Opel, and is also working toward an alliance with Chrysler in deals that would create the world's biggest carmaker.
The alliances are intended to help the companies to weather the downturn in global auto markets.
But the Fiat workers fear the agreements could lead to plant closures and job losses.
Trade unions claimed 15,000 workers gathered at Fiat's headquarters in the northern Italian city, while police put the total nearer 3,000.
Fiat has said it will try to preserve as many jobs as possible in Italy.