Denmark is holding two people in custody and four others are the target of a terrorism investigation, a prosecutor has said.
The case coincides with one arrest in the Netherlands and several in Germany of alleged Hamas members.
Authorities in Germany said three people arrested there were suspected of preparing for attacks on Jewish institutions in Europe.
Danish authorities say that one person was arrested in the Netherlands but it was not clear if there were any ties to the Hamas investigation in Germany.
Denmark has not cited an alleged Hamas link in its investigation.
The two people being held in Denmark were ordered to remain in pretrial detention until January 9.
Danish media identified them as a man in his 50s and a 19-year-old woman.
Danish intelligence agency PET on Thursday announced the arrests of three people on suspicion of plotting to carry out “an act of terror”.
One of them, identified by Danish media as a 29-year-old man, was released, prosecutor Anders Larsson said early on Friday after a night-long custody hearing at a Copenhagen court.
Mr Larsson also said that four other people were held in “pretrial custody in absentia” but did not say whether authorities knew their whereabouts or if an active search for them was under way.
Without elaborating, he said there was “still someone at large”.
None of the suspects can be identified because of a court order and the custody hearing was held behind “double closed doors” — meaning no details were available about the case, which is shrouded in secrecy.
It was not immediately clear if and how the Danish and the German arrests were connected.
German prosecutors allege that the three men detained in Germany on Thursday were tasked with finding a previously set-up underground Hamas weapons cache in Europe.
“The weapons were due to be taken to Berlin and kept in a state of readiness in view of potential terrorist attacks against Jewish institutions in Europe,” they said.
Two men were held in Berlin, while a third suspect was temporarily detained in Berlin, Germany’s federal prosecutor said, adding that one also was taken into custody in the Dutch port city of Rotterdam.
In line with Germany privacy rules, authorities only identified the men by their first names and the first initial of their last name: Abdelhamid Al A, born in Lebanon; Egyptian national Mohamed B; Dutch national Nazih R and Ibrahim El-R, born in Lebanon.
The authorities alleged the suspects “have been longstanding members of Hamas and have participated in Hamas operations abroad”.
They said the suspects were closely linked to the leadership of Hamas’s military wing, which is considered a terrorist organisation by the United States and the European Union.
On Friday, a judge ordered the three men detained in Berlin to be held in custody pending a possible indictment for being members of a foreign terrorist organisation, prosecutors said.
Earlier this month, the EU’s home affairs commissioner, Ylva Johansson, said Europe faced a “huge risk of terrorist attacks” over the Christmas holiday period amid the Israel-Hamas war.
In Brussels, where she attended a European Union summit, Danish Prime Minister Mette Frederiksen mentioned the Danish, German and Dutch cases but declined to tie them together.
She said the wider picture for security in Europe was worrying.
“We have seen how ships are attacked in the Red Sea off Yemen,” she told a press conference in reference to a ballistic missile fired by Yemen’s Houthi rebels that hit a cargo ship on Friday in the Red Sea, following another attack only hours earlier that struck a separate vessel.
“Individually, these incidents are serious and worrying, but together they paint a picture of something bigger. That we are facing a more serious and complex threat picture,” she said.
“It is very, very serious.”