The group, acting in Russia's interests, planned the sabotage acts until at least Oct. 16, 2025.
The suspects intended to ship parcels containing explosives and incendiary materials from Poland to Ukraine, designed to detonate or catch fire during transit, according to the probe.
Prosecutors said the goal was to spread fear among the population and destabilize EU nations supporting Ukraine. The parcels were intercepted by Romanian intelligence before they could explode.
Polish authorities arrested 21-year-old Ukrainian citizen Danylo H., who lived near Warsaw and worked at a company warehouse. He faces charges of aiding foreign intelligence and preparing terrorist acts.
A Warsaw court ordered his three-month pre-trial detention. Investigators said he worked with accomplices carrying out Russian special services' tasks.
Two other Ukrainian citizens cooperating with Danylo H. were detained in Romania, where a Bucharest court mandated their 30-day arrest.
Poland's special services coordinator spokesman Jacek Dobrzynski said Danylo H. and others arranged a route to transport the explosives through Poland and Romania to Ukraine.
Over recent months, Poland's Internal Security Agency has detained 55 people working for Russian intelligence or against Polish security.
On Oct. 13, Polish authorities charged a Russian couple, Igor and Irina R., with spying for Russia's FSB and preparing explosives meant for delivery via courier parcel.
Investigators found the pair gathered intelligence on Russian opposition figures in Poland and, with others, helped send a parcel with explosive materials that special services intercepted in time.