Three people were arrested in Denmark and one in the Netherlands on suspicion of preparing a terrorist attack, Danish police said on Thursday.
They added that the moves followed a coordinated cross-border police effort.
There was no word yet on what the suspects' motives were or whether they were related to militancy or to far-right groups.
"The investigation has revealed that a network of people has been preparing a terrorist act," Flemming Drejer, the Danish Security and Intelligence Service PET's chief superintendent, told a press briefing in Copenhagen.
"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," he said.
Dutch police said a 57-year-old man was arrested in Rotterdam on Thursday on request of German authorities in a Danish-German investigation.
The Dutch National Coordinator for Counter-terrorism and Security (NCTV) this week raised the country's threat level to 4, or "substantial" on its 5-step scale, citing the conflict between Israel and Hamas.
Danish police said raids were ongoing across the country and were carried out at an early stage of the investigation.
The network had links to organised crime both in Denmark and abroad, including to a gang named Loyal To Familia, or LTF, Dreyer said, but declined to elaborate on possible motives.
Police said they would increase their public presence in coming days, in particular in Copenhagen and around Jewish localities.
The Jewish Community in Denmark said it had been briefed before the public about the raids but did not have any knowledge about concrete threats to Jewish targets.
"This is extremely serious," Danish Prime Minister Mette Frederiksen told news agency Ritzau at a meeting with EU leaders 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. Who are against our freedom and who are against Danish society, with all that it entails," Frederiksen said.
PET has for more than a decade warned against potential attacks. Police said they had kept unchanged their terrorist alert level at 4 on a scale of 1-5, reflecting a "significant" threat.
The three arrested in Denmark would be charged under the terrorism clause of the criminal code and put in front of a judge for preliminary questioning, police said.