Danish police said they prevented a terror attack on Thursday after announcing that four people had been arrested.

Three suspects were arrested in Denmark, and a fourth was arrested in the Netherlands.

"The investigation has revealed that a network of people has been preparing a terrorist act," said Flemming Drejer, chief superintendent of the Danish intelligence agency, PET.

"The arrests and the raids we're carrying out today are based on an intensive investigation that PET has carried out in close cooperation with our partners abroad," Drejerm said.

Police raids were ongoing across the country, he added.

What do we know about the alleged terror plot?

Drejer said the suspects would face a custody hearing within 24 hours, likely behind "double closed doors," meaning that he could not give details about the case, any target or motive.

He said the arrests were "carried out in close collaboration with our foreign partners" and that police had "a special focus" on Jewish institutions.

The office of Israeli Prime Minister Benjamin Netanyahu later said the suspects were operating "on behalf of" Hamas, which is considered a terror group by the European Union and several other governments.

"This is extremely serious," Danish Prime Minister Mette Frederiksen on the sidelines of the year's final EU summit in Brussels.

"It is of course — in relation to Israel and Gaza — completely unacceptable for someone to bring a conflict elsewhere in the world into Danish society," she said.

"For a number of years now, we have seen that there are people living in Denmark who do not wish us well," she added, "who are against our freedom and who are against Danish society, with all that it entails."

Later on Thursday, German prosecutors announced three arrests of suspected Hamas members in Berlin and another in Rotterdam. Danish security officials said there is no direct link between the two sets of arrests.

zc/lo (Reuters, AFP, AP)