Cops conducted a raid on a German refugee centre in a search for four men suspected of links to ISIS killers and possibly preparing an attack, it has been reported.
Officers swooped on sites in Berlin and across northern Germany as fears over a possible Islamist attack remain high.
German media reported police as saying that the main suspect was arrested in a shelter for migrants in Attendorn.
In close cooperation with the domestic intelligence agency, they arrested two men and one woman on existing warrants related to other matters, they said.
German media reported that those arrested were Islamists.
Fears about an attack on German soil have risen since deadly bombings and shootings in Paris in November.
Today’s raids, carried out in part by special forces, took place on four flats and two offices in Berlin and also in properties in North Rhine-Westphalia and Lower Saxony.
Police seized computers, mobile telephones and sketches.
One of the suspects is wanted by Algerian authorities, who believe he is a member of Islamic State, said the Berlin police.
He is suspected of having military training in Syria.
Police in Dortmund said they had conducted an operation in the small town of Attendorn in North Rhine-Westphalia but a spokesman declined to give further details.
German authorities have repeatedly warned of the threat of attacks.
They cancelled a friendly international soccer match in Hanover last year and closed stations in Munich at New Year due to security concerns.