Two suspects were also arrested in Romania for allegedly depositing incendiary devices at an international courier company in Bucharest.
Authorities in Poland have arrested eight people across the country on suspicion of preparing acts of sabotage, a spokesperson for the National Prosecutor's Office has said.
Prosecutors have accused one of the detainees, who is a Ukrainian national, of being involved in plans to send explosive materials to Ukraine.
Two other Ukrainians, aged 21 and 24, are also thought to have been involved in the plot, which was allegedly orchestrated by the Russian intelligence services.
The pair were detained in Romania, the Romanian authorities confirmed.
They were arrested in Romania after depositing two parcels containing improvised incendiary devices at an international courier company in Bucharest, according to the Romanian authorities.
Prime Minister Donald Tusk posted about the detention of eight people on X on Tuesday morning.
"The case is ongoing. Further operational activities are continuing," he said.
The goal of their actions, Polish prosecutors said, was to intimidate populations and destabilise EU countries supporting Ukraine.
Tomasz Siemoniak, Poland's minister coordinating special services, also wrote on social media that the detainees were arrested on suspicion of "conducting reconnaissance of military facilities and critical infrastructure, preparing resources for sabotage and directly carrying out attacks".
Since Russia’s full-scale invasion of Ukraine in February 2022, Poland has detained dozens of people over suspected sabotage and espionage.
Last month, prosecutors in Poland's Baltic neighbour Lithuania said that they had uncovered and detained a Russia-linked network of suspects alleged to have planned and organised arson attacks in various European countries.
Western officials have accused Russia and its proxies of being behind dozens of incidents across Europe since the invasion of Ukraine three years ago.
The suspected attacks range from stuffing car exhausts with expanding foam in Germany to a plot to plant explosives in cargo planes.