Marcin Kierwiński told public broadcaster TVP Info on Sunday that the man appeared to belong to "very radical" groups and had made contact with like-minded people online.
He described the suspect as highly active in "communities that promote extreme views, use hateful language and call for violence and hatred in public life."
Investigators traced the man, who was detained in Poland's southwestern Lower Silesia region, by closely analysing content he had posted online, Kierwiński said.
The lead came from an informant who contacted the State Protection Service (SOP) on Sunday with details on who was allegedly behind the planned attack.
Kierwiński declined to speculate on whether the man had a political motive, saying that would have to wait until police complete their work.
Investigators are still examining the detainee's contacts and online activity, and further decisions will depend on the findings, he said.
Tusk and Kosiniak-Kamysz, who also serves as a deputy prime minister, thanked the security services for their swift response.
Interior ministry spokesman Jacek Dobrzyński said the information was treated with the utmost seriousness and made public because "it was credible, and to prevent speculation."
He said "threats against Poland and Poles" were rising year on year, and this time they targeted the country's most senior officials.
(ał/gs)
Source: PAP