Dutch police have arrested 12 Somali men in the port city of Rotterdam on suspicion of preparing an attack, according to the public prosecutor.
The men, aged 19 to 48, were detained on Friday on a tip from AIVD, the Dutch intelligence and security service, that they were planning an attack shortly in the Netherlands, prosecutors said in a statement.
"The [AIVD] message reports that a number of Somalis wanted to make a terrorist attack in the Netherlands in the short term."
A telephone shop and four houses in the city of Rotterdam were searched, as well as two motel rooms in Gilze-Rijen, a village in the south of the country, the prosecutors said on 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.
Prosecutors must bring the suspects before a judge by Tuesday or release them.
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.
The Dutch National Terrorism Co-ordinator left the terrorist alert level unchanged following the arrests, indicating the likelihood of an attack was "limited".
There was no 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.
Dutch intelligence services reportedly have been closely watching the growing Somali community in the Netherlands. One US citizen of Somali extraction is under arrest and is fighting extradition to the US, suspected of supplying money to the al-Shabab group for weapons and to finance trips for potential recruits.
The US state department considers al-Shabab a terror group with links to al-Qaeda.
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 jet 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 this week in Rome.
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.