The arrests were part of "an inquiry into international jihadist terror," a spokesman for Belgium’s federal prosecution office said. The alleged extremists were plotting an attack in Belgium, the prosecutor's office said.
"In total 10 people suspected of preparing an attack in Belgium were arrested in Belgium, Holland and Germany," he said.
The target of the plot had "not been determined yet" when the raids took place, the prosecutor's office said in a statement.
The suspects are from Belgium, the Netherlands, Morocco and Chechnya, the statement said. Most live in Antwerp.
The arrests came as Germany remained on high alert amid fears of a terrorist attack. Interior Minister Thomas de Maizière stressed that authorities were watching the “potentially dangerous people.”
“We know rather a lot,” he said on Monday night to broadcaster ARD. Authorities were “not so naïve” as was the case at the time of the attacks on the United States on September 11, 2001, he added.
However security services are now particularly worried about what they call the “quiet observers” among Islamists in Germany, sources told news agency DAPD.
“They could exploit our security gaps and therefore, despite all our efforts at defence, hit us with attacks,” a security source said.
Among the hundreds of people regarded as “dangerous” are “experienced observers” who “are the best informed about the weak points in our security architecture,” unnamed security specialists said.
In the Belgian led arrests, the alleged extremists used the website Ansar Al Mujahideen as part of its plot. The arrests followed a months-long investigation that was launched by authorities in the northern Belgian city of Antwerp in late 2009, the statement said.
The investigation focused on recruiters, would-be "jihadists" and the financing of a Chechen "terrorist organisation," it said.
Several other people have already been arrested in Spain, Morocco and Saudi Arabia as part of the probe, the Belgian authorities said.
The investigation was conducted in collaboration with several countries and the European Union's judicial cooperation unit Eurojust.
Europe has been on high alert for several weeks over heightened fears of terrorist attacks. Western security officials have warned that al-Qaida may be planning attacks in Europe similar to those in the Indian city of Mumbai in 2008.
The United States issued a travel alert on October 3 for its citizens travelling in Europe, citing the risk of potential terrorist attacks on transportation systems and tourist attractions.
Similar alerts were issued by Japan, Sweden, Britain and France. A plot to blow up cargo planes was uncovered at the end of last month after booby-trapped parcels were found at airports in Dubai and Britain.