Some 100 security workers walked off the job at 5 am and did not return for several hours.
The workers are trying to pressure employers to increase wages for some 22,000 security workers in the state of North Rhine-Westphalia.
According to two-year labour contracts, employers offer an hourly pay raise of €0.13 for 2009, and €0.11 for 2010. But with the lowest hourly rate for these workers at €7.53, and the highest at €12.38, this offer can only be seen as a provocation, the union said.
Verdi also called on workers at the Solvay chemical plant in Rheinberg to strike.