Amazon workers in a number of European countries are taking industrial action to coincide with the internet retail giant's Prime promotion event.
On Tuesday, thousands of staff at warehouses in Germany will walk out to demand better conditions, joining those taking action in Spain and Poland.
Germany is the company's second biggest market after the US.
The action adds to Amazon's Prime Day difficulties. On Monday, some shoppers struggled to access its website.
Germany's Verdi services union said the company was getting rich by "saving money on the health of its workers".
But Amazon said its workers were paid fairly, with permanent staff earning €12.22 (£10.81) an hour or more after two years.
Amazon said it expected only a fraction of its 12,000 workers in Germany across its six sites to join the strike. It said deliveries would be unaffected.
The Verdi union also said workers in Spain were staging a three-day strike, while Polish workers were staging a work to rule.
On Monday, many Amazon customers reported that the site had crashed, showing them only an error message that read: "sorry, something went wrong on our end".
The issues centred on the US but occurred on other continents too.
DownDetector.com, which tracks outages, said the problems began shortly after the sale kicked off at 15:00 on Monday in the US.
Amazon launched Prime Day in 2015 and by 2017 it was its second biggest shopping day, topped only by Cyber Monday.