Police in central Denmark early Tuesday arrested nine men suspected of plotting a terrorist attack and acquiring material to build explosives, the intelligence service said.
Justice Minister Lene Espersen called the case "very serious," and said the group was likely planning an attack in Denmark.
Investigators said they were not sure how far the plans had progressed but decided to launch a pre-emptive strike after keeping the group under surveillance for some time.
"It is among the worst that has happened in Denmark," Espersen told the TV2 channel, but did not give any details.
The sweep was carried out at 2 a.m. in Vollsmose, a mostly immigrant suburb west of Odense, Denmark's third-largest city.
Explosive materials
Lars Findsen, the head of the Danish Security Intelligence Service, said the suspects "had acquired material ... to build explosives in connection with the preparation of a terror act."
He said the suspects — both ethnic Danes and people with immigrant backgrounds between the ages of 18 and 33 — would face a custody hearing later Tuesday in Odense, 100 miles west of Copenhagen.
Findsen did not reveal the planned target of the attack and said it was hard to evaluate how far the suspects had progressed in their preparations.
"With the general terror situation, the Danish Security Intelligence Service didn't want to run any unnecessary risk," he said.
Denmark raised its terror preparedness level after attacks in recent years in London and Madrid and the global Muslim fury earlier this year over caricatures of the Prophet Muhammad first published in a Danish newspaper.
Tuesday's arrests would not prompt the Scandinavian country to raise its terror alert further, Findsen said. He also said the sweep was not related to a terror investigation in Germany in which four Lebanese suspects are being held in connection with a failed train bombing attempt.
German media claimed one of the suspects, Youssef Mohamad el Hajdib, who was arrested Aug. 19 in the northern German city of Kiel, was heading to Denmark. German and Danish media reported German police found a telephone number in his pocket for an imam living in Odense.
Two weeks ago, four suspects in custody in Denmark since October were charged with supplying explosives for a planned terror attack in Europe. Police in Bosnia say they foiled the plot when they arrested two men accused of preparing to blow up an unspecified target.
Terrorists have not hit Denmark in 20 years, but the July 2005 bombings in London stirred fears that the Scandinavian country could be targeted for its participation in the U.S.-led coalition in Iraq.
"We have seen the faces of terror in Madrid and London. We know that there are potential terror threats in the whole Western world," Denmark's Deputy Prime Minister Bendt Bendtsen told DR public television.
In 1985, a bomb exploded outside the offices of North West Orient airlines in Copenhagen, killing one person and wounding 16. Three Palestinians living in Sweden were convicted of planting the bombs and sentenced to life in prison in 1989.
