WARSAW, Poland -- An undercover television report that exposed Polish neo-Nazis dressed in Nazi uniforms celebrating Adolf Hitler's birthday and burning a swastika has prompted Poland's prime minister to denounce fascism and authorities to launch an investigation.

The report was broadcast by TVN24 on Saturday and revealed the same neo-Nazi group, Pride and Modernity, was behind a November protest in Katowice in which participants hung pictures of six centrist Polish European Parliament lawmakers on mock gallows.

The prosecutor general launched an investigation Sunday into the "public propagation of fascism," a crime that can carry a sentence of up to two years in prison upon conviction.

Prime Minister Mateusz Morawiecki says propagating fascism tramples "the memory of our ancestors and their heroic fight for a Poland that is just and free from hatred."