Verdi also filed a lawsuit against a decision by regional authorities to allow Amazon staff in two German cities to work this Sunday as the mail-order group steps up efforts to deliver orders to customers before the Christmas holiday.
Industrial action this week had already been extended until Saturday in four of Amazon's nine distribution centres in Germany and until December24 at one warehouse.
More than 2,400 workers took part in walkouts on Friday, Verdi said.
The union has organised frequent strikes at Amazon in Germany since May 2013 as it seeks to force the retailer to raise pay for warehouse workers in accordance with collective bargaining agreements across Germany's mail order and retail industry.
Verdi said that the lawsuit filed against Amazon meant that the authorities' approval of the group putting its employees to work this Sunday was effectively void.
"As the Federal Administrative Court only recently stated, work on Sunday has to be reserved for strictly exceptional cases, which we do not see in the case of Amazon," Verdi board member Stefanie Nutzenberger said in an emailed statement.
Amazon, which was not immediately available for comment, has repeatedly rejected the union's demands, saying it regards warehouse staff as logistics workers and that they receive above-average pay by the standards of that industry.
The U.S. company employs almost 10,000 regular staff at its warehouses in Germany, its second-biggest market behind the United States, as well as more than 10,000 seasonal workers. It can also draw on 19 other warehouses across Europe.
Amazon said on Wednesday that its deliveries had not been delayed by industrial action so far and that it had even extended to midday on December 22 the order deadline for gifts to reach customers in time for December 24 by normal delivery.
© Thomson Reuters 2014