Frankfurt - Hundreds of workers at Amazon's German operations went on strike on Monday, just as pre-Christmas sales were set to peak.
They are embroiled in a dispute over pay that has been raging for months.
Germany is Amazon's second-biggest market behind the United States and sales there grew almost 21% in 2012 to $8.7bn, a third of its overseas total.
Amazon took its most daily orders in Germany on 16 December, with shipments peaking on 17 December.
Amazon, which employs 9 000 warehouse staff in Germany as well as 14 000 seasonal workers at nine distribution centres, said only 640 workers had failed to turn up for the early shift so there had been no delays to deliveries so far.
Performance pressure
"Our customers can continue to rely on us for the prompt delivery of their Christmas presents," a spokesperson said.
The Verdi union said up to 700 workers joined the strike in Amazon's logistic centre in Bad Hersfeld, plus more than 200 in Leipzig. The union also called a strike in Graben for the first time and it said it had a very good turnout.
Verdi board member Stefanie Nutzenberger said: "The Amazon system is characterised by low wages, permanent performance pressure and short-term contracts."
A delegation of German workers will also protest at Amazon's headquarters in Seattle, helped by US unions. In addition, workers in Amazon's centre in the German town of Werne will strike on Tuesday, Verdi said.
Unskilled staff
Kleber said Amazon pays warehouse workers well according to the standards of the logistics industry and does not think the more generous terms of the mail order and retail sector are justified for its largely unskilled staff.
Amazon has recently announced it will build three new logistics centres in Poland and two in the Czech Republic, prompting speculation that it could seek to shift work across the border from strike-hit centres in Germany.