Dutch security forces arrested four people in the port city of Rotterdam on Saturday on suspicion of preparing for a “terrorist crime”, police said, while a fifth suspect was detained in Germany.
Elite counter-terrorism units and police arrested the men and carried out searches at the locations where they were detained, Rotterdam police said.
“They are suspected of involvement in the preparation of a terrorist crime,” a brief police statement said.
“Investigations will continue in the coming days, with the emphasis on further investigation of the nature and scale of the terrorist threat.”
In Germany, police in Mainz said they arrested a 26-year-old Syrian man following an extradition request from the Dutch authorities.
“He is strongly suspected of taking part in the preparation for an attack in the Netherlands,” they said.
Further details on the suspects and the crime they were allegedly preparing for were not immediately available.
The Netherlands has been largely spared the kind of terror attacks which have rocked its closest European neighbours in the past few years, but there has been a series of recent scares.
In August a 19-year-old Afghan with a German residence permit stabbed and injured two American tourists at Amsterdam’s busy Central Station before being shot and wounded.
In September Dutch investigators said they had arrested seven people and foiled a “major attack” on civilians at a major event in the Netherlands.
They said they had found a large quantity of bomb-making materials including fertiliser likely to be used in a car bomb.
In June two terror suspects were arrested while close to carrying out attacks including at an iconic bridge in Rotterdam and in France, prosecutors said.
The men aged 22 and 28, who are of Moroccan origin, made a film at the Erasmus bridge in which they sang a martyrdom song, they said.