General Motors plants across Europe protested at job cuts at the Opel car factory in Antwerp.
The European Metalworkers' Federation and a group representing union representatives from GM plants said all the carmaker's factories in Europe took part in demonstrations - which included work stoppages - starting in the morning.
The Opel plant in Antwerp is on strike until Tuesday to protest plans to lay off 1,400 workers and reduce production under company restructuring plans.
In a note published on the union's blog, the unions said there would be protests at Opel, Powertrain, Vauxhall, and IBC plants in Germany, France, Hungary, Sweden, Britain, Austria, Spain and Russia.
Posts on the Web site said a factory at Luton, near London, England, stopped work during the morning shift.
The site also showed photos of a protest outside Opel's Kaiserslautern plant in Germany, saying more than 2,500 people took part.
GM's three Polish factories did not join in only because May 3 is a national holiday, the unions said.
The Opel plant in Belgium, which employs 5,100 people, is facing a phaseout of production of the Astra model under plans announced by GM last month. The next generation of the Astra will be built at plants in Germany, Sweden, Poland and Britain.
The company said it would like to shift Antwerp's production to building 80,000 Chevrolet model cars, and possibly the production of another unspecified 60,000 cars of another model.
Unions, however, said they would demand higher production quotas to save jobs at the plant, which makes about 220,000 cars annually.