"If someone wants to increase market share, they have to take it away from someone else. Each piece of the cake taken by Amazon, will be at the cost of Allegro, but the e-commerce market will grow as well," said Konrad Ksiezopolski, analyst at Haitong Bank.
Allegro, whose shares fell by 6.3% by 1220 GMT, commented to Reuters that it had been growing customer numbers while competing with Amazon for several years.
Amazon did not say when it will launch the Polish shopping website and whose lockers for parcel storage it would use.
"At the start Amazon may cooperate with InPost, there was information about Polish Post too. I assume though that ultimately it will rather grow its own locker network," said Tomasz Sokolowski, analyst at Santander DM.