AMSTERDAM - Dutch police 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, ages 19 to 48, were detained Friday on a tip from 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's Schiphol airport on Christmas Day and allegedly tried to blow up a plane over Detroit with explosives taped to his underwear.

Holiday security concerns in Europe also have increased after 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.

Dutch intelligence services reportedly have been watching the growing Somali community in the Netherlands. One U.S. citizen of Somali extraction is under arrest and fighting extradition to the United States, 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-Qaeda.