Belgian anti-terrorism investigators on Friday were holding one alleged member of an extreme right group suspected of preparing bomb attacks and inciting violence.
The Belgian probe also found evidence allowing police in Germany, Italy, Croatia and Lithuania to carry out raids that may lead to further arrests.
The investigation was launched in May and house raids were carried out on Thursday in the North Sea port of Ostend and in Diepenbeek in the east of the country.
"During one of the house searches in Diepenbeek, large numbers of Nazi memorabilia, including Nazi flags and swastikas, were found," prosecutors said in a statement Friday.
In the searches, the statement said, a man and a woman in their early 20s were detained and identified as Daan C. and Kayley W. from the Dutch-speaking region of northern Belgium.
The male suspect, Daan C., was found at the house with Nazi symbols, and he has been remanded in custody under an arrest warrant pending further investigation.
Kayley W. was freed after questioning.
"Both persons are suspected of participating in the activities of a terrorist group," the statement said, with Daan C. accused of being a leader.
Prosecutors accuse him of "recruiting people with a view to committing terrorist crimes and preparing to commit terrorist crimes."
But a source close to the inquiry told AFP on condition of anonymity that no evidence was found of specific, imminent planned attacks.
Using encrypted online platforms, group members shared manuals for 3D-printed guns and shared images appearing to show pipe bombs and Molotov cocktails.
"It also turned out that some members of the online groups had written a manifesto, and there are indications that they had weapons at their disposal," the federal prosecutor's office said.
