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 this week in Rome.

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.