WARSAW, Poland — An undercover expose of Polish neo-Nazis celebrating Adolf Hitler in a nighttime forest ceremony has sparked revulsion in Poland, a country where Nazi Germany murdered millions of people in concentration camps, ghettos and the bombing of cities.

In the week since the television expose was broadcast, political leaders in Poland have condemned the extremists. An investigation has led to the detention of six people and to the confiscation of fascist paraphernalia and ammunition.

There is widespread disbelief that young Poles could celebrate Hitler, who believed that Poles and other Slavs were “Untermenschen,” or subhuman, and forced many into labor for Nazi Germany or put to death.

The debate was sparked by a report broadcast Jan. 20 by private news station TVN24 that showed members of a Polish neo-Nazi group, Pride and Modernity, celebrating the 128th anniversary of Hitler’s birth in a forest at night last spring.

Using hidden cameras, TVN24 captured footage of them preparing for the ceremony by dressing in SS and Wehrmacht uniforms and nailing swastikas to trees.

Mostly young men, they raised a toast to Hitler, whose photo also hung from a tree, praising him and making the stiff-armed “Sieg Heil” salute as a large wooden swastika burned. The ceremony included a birthday cake decorated with a swastika.

The ceremony took place 33 miles (53 kilometers) from Auschwitz, where Nazi Germany killed more than a million people, most of them Jews from across Europe but also many Poles. On Saturday a ceremony will be held at the former death camp to mark the 73rd anniversary of its liberation by Soviet troops.

The denunciations from leaders came swiftly.

Prime Minister Mateusz Morawiecki called this week for tougher penalties against individuals and organizations convicted of propagating fascism, which he said tramples “the memory of our ancestors and their heroic fight for a Poland that is just and free from hatred.”

“Organizations that are using symbols and concepts that glorify and tolerate German Nazism or other totalitarianisms should be outlawed,” Morawiecki said.

President Andrzej Duda, asked about the matter while at the World Economic Forum in Davos, said such behavior “must be eradicated with all of our might.”

“There is no place in Poland for the glorification of Adolf Hitler, on whose orders six million Polish citizens were murdered,” Duda said.

Interior Minister Joachim Brudzinski also strongly condemned the extremists, telling lawmakers in parliament on Thursday that “you have to be a complete idiot to dress up in an SS man’s uniform and eat a waffle cake with a swastika in the forest. We will crack down on this!”

But he also faced criticism for seeming to portray them as a marginal phenomenon, even though sociologists and other experts have long been warning of a rise of far-right extremism in Poland.

“In Poland, such toads have to hide in the woods, but in Germany they walk through the center of Berlin,” Brudzinski told lawmakers.

Some members of the opposition Civic Platform described his words as a disgrace.