Danish authorities said on Thursday they prevented a terror attack, after three arrests in Denmark and a fourth in the Netherlands.
Officials did not go into details of the suspects or give any indication as to the possible target of the alleged plot.
"It was a group that was planning an act of terror," Flemming Drejer, head of operations at the PET intelligence service, told a news conference.
There were "ramifications involving other countries" and organised crime, he added.
Drejer said other suspects currently abroad were also thought to be implicated in the plot.
The PET and several police districts made the arrests in Denmark in early morning raids in several parts of the Scandinavian country, the officials said.
The PET considered the threat to be critical, putting it at four on their five-point threat scale.
Police stepped up their presence in Copenhagen but said the capital remained "safe".
Prime Minister Mette Frederiksen said the operations "show us the situation that Denmark is in".
"For several years we have noted that there are people who live in Denmark and who do not wish us well, who are against our democracy, our freedom, and who are against Danish society," she told reporters.
- Anti-Danish sentiment -
Over the summer, Denmark and neighbouring Sweden became the target of anger in several Muslim countries after a slew of protests in Scandinavia involving burnings and desecrations of the Koran.
In Iraq, nearly a thousand protesters attempted to march on the Danish embassy in Baghdad's fortified Green Zone in late July following a call by firebrand cleric Moqtada Sadr.
Between July 21 and October 24 this year, 483 book burnings or flag burnings were recorded in Denmark, according to national police figures.
In response Denmark's parliament adopted a law earlier this month that criminalises the burning, tearing or otherwise defiling of religious texts such as Islam's holy book.
In 2006, a wave of anti-Danish anger and violence erupted in the Muslim world following the publication in the small Nordic country of cartoons of the Prophet Mohammed.
And in February 2015 a gunman who had voiced allegiance to the Islamic State (IS) group opened fire at a cultural centre in Copenhagen that was hosting a forum on Islam and free speech.
Last year, a Danish court sentenced an IS sympathiser to 16 years in jail for plotting a bomb attack. The verdict was the most severe ever handed down under Denmark's anti-terrorism laws.
The defendant pleaded not guilty, saying the 12 kilogrammes (26 pounds) of powder and chemical substances concealed at his home were intended for manufacturing fireworks.