Those targeted in the sweeps were suspected of planning a possible attack in Belgium or of involvement in recruiting for an alleged Chechen terror organization, Belgian officials said. Seven were detained in Belgium's port city of Antwerp, three in Amsterdam and one near the German border city of Aachen.
The arrests were not linked to the recent reports of possible terrorist attacks in Germany, said Judith Sluiter, a spokeswoman for the Dutch National Coordinator for Counterterrorism.
In a second spate of 17 house searches in and around Brussels, "several people" were interrogated in an unrelated investigation into an alleged terror group seeking funds and recruits to go fight in Iraq and Afghanistan, said Leen Nuyts, spokeswoman for the Belgian prosecutor's office.
She refused to be more specific about the number of detentions but insisted the second case had nothing to do with a potential terror attack plot.
In the first sweep, ten homes were searched in the three nations on Tuesday morning, and 11 suspects of Belgian, Dutch, Moroccan or Russian nationality were detained, all men in their twenties or thirties, Nuyts said. Initially there were indications one woman was among them.
They follow arrests in Spain, Morocco and Saudi Arabia, related to this investigation earlier this year.
The Belgian prosecutors said "there was talk of plans for an attack in Belgium by an international jihadist organization" that uses the website Ansar al Mujahideen. The place of the alleged attack had not been specified.
The police also targeted "the recruiters, candidate jihadists and financing" for the Caucasus Emirate, which groups insurgents who seek to establish an Islamic emirate in Russia's North Caucasus region of Chechnya, Dagestan and Ingushetia. Its leader is Chechen rebel Doku Umarov.
In Antwerp, Mohammed Hamdaoui, the brother of two alleged radicals who were detained on Tuesday, denied they could be linked to terror activities.
"They go nowhere, they haven't been on a big trip, not even to Morocco," he said.
Germany's Federal Criminal Police confirmed they arrested a 31-year-old man who was sought on an EU arrest warrant in connection with suspicion of recruiting young men in Belgium to fight in Chechnya.
In a statement, Dutch prosecutors said they had detained three men aged 25, 26 and 28 in Amsterdam at the request of Belgian authorities on suspicion of involvement in international terrorism.
The Dutch National Prosecutor's Office said Austria was also involved in the action.
Dutch authorities said Belgium had asked for the extradition of the three suspects arrested in Amsterdam.
Separately, the Danish Security and Intelligence Service said in its annual assessment of the terror threat that the Scandinavian nation is a "high-priority terrorist target" because of newspaper cartoons of the Prophet Muhammad that sparked riots in the Muslim world in 2006. Individuals and locations linked to the cartoon case are specifically at risk, it said.
It noted a series of recent attempts to carry out attacks in Denmark, including the September arrest in Copenhagen of a Chechen man who accidentally set off a letter bomb that the agency believes was intended for the Jyllands-Posten newspaper that first published the 12 cartoons.