AMSTERDAM -- Dutch police have arrested 12 Somali men in the key port city of Rotterdam on suspicion of preparing a terrorist attack, the public prosecutor said Saturday.
The men, aged 19 to 48, were detained Friday on a tip from the intelligence services that they were planning an attack shortly in the Netherlands. There was no immediate information on the alleged target, but Rotterdam is Europe's biggest port and a hub of maritime commerce, with huge oil and gas storage facilities and dozens of massive docks.
European officials stepped up security around the holidays this year after a Nigerian man in 2009 left Amsterdam airport on Christmas Day and allegedly tried to blow up a plane over Detroit with explosives taped to his underwear.
There also have been growing holiday security concerns in Europe following a suicide bombing in Sweden and attacks on two embassies last week in Rome.
Dutch police searched an Internet cafe, four houses and two motel rooms in the Rotterdam area, prosecutors said Saturday. No weapons or explosives were found. Six of the suspects lived in Rotterdam, five had no permanent residence and one came from Denmark, they said.
Asked how serious the threat was, a senior prosecutor said the intelligence tip warranted action. "It's uncertain whether we escaped from an attack. What we did is take away the threat that was formed by these people," Gerrit van der Burg said on national NOS television.
Prosecutors must bring the suspects before a judge by Tuesday or release them.
The Dutch National Terrorism Coordinator left the terrorist alert level unchanged following the arrests, indicating the likelihood of an attack was "limited."
Dutch intelligence services reportedly have been closely watching the growing Somali community in the Netherlands. One U.S. citizen of Somali extraction is under arrest and is fighting extradition to the U.S., suspected of supplying money to the al-Shabab insurgent group for weapons and to finance trips for potential recruits. The U.S. State Department considers al-Shabab a terror group with links to al-Qaida.
Heightened nervousness of a holiday terrorist attack has led to mistakes in the past. Three months ago, police arrested two Yemenis traveling from the U.S. on a request from U.S. law enforcement agencies who feared they were conducting a dry run for a terrorist attack. They were released two days later for lack of any evidence of a crime.
Last year, 24-year-old Umar Farouk Abdulmutallab, who had studied in London, boarded a Northwest Airlines flight to Detroit from Amsterdam. He is accused of trying to blow up the flight, and a judge in a federal court in Detroit has entered a not-guilty plea on his behalf.
On Thursday, anarchists sent mail bombs to the Chilean and Swiss embassies in Rome, injuring two mail employees. A top Italian security official said the attackers wanted to avenge blows by those countries against their movement.
Last Monday, 12 men were arrested in Britain in the largest counterterrorism raid there in nearly two years. The men -- whose ages ranged from 17 to 28 -- were arrested in London, Cardiff, Stoke-on-Trent and Birmingham. At least five were of Bangladeshi origin.
Security officials said a large-scale terror attack was aimed at British landmarks and public spaces. Lord Carlile, the U.K. government's independent watchdog for terror, said the alleged plot appeared significant and involved several British cities, but he did not identify the targets.
Police removed computers from the suspects' homes and have up to 28 days to either charge the men or release them.
French officials, meanwhile, have ordered plainclothes police patrols in key tourist sites for the holidays, including an extra 6,000 more police for New Year's Eve.
Europe has been the target of numerous terror plots by Islamist militants. The deadliest was the 2004 Madrid train bombings, when shrapnel-filled bombs killed 191 people and wounded about 1,800. A year later, suicide bombers killed 52 commuters in London on three subway trains and a bus.
In 2006, U.S. and British intelligence officials thwarted one of the largest terror plots yet, a plan to explode nearly a dozen trans-Atlantic plane.