An ever-increasing number of extremist groups have taken root in Brussels. In March, terror attacks across the city killed 32 people.
In the past two years Belgium authorities foiled six terror attacks, its head of judicial police Eric Jacobs in Brussels said in an interview published in the daily "La Dernieure Heure" on Tuesday.
Jacobs told the daily publication that the number of potentially violent groups had multiplied in recent years.
"There are more than 200 nationalities in the capital. There is not only Daesh [an acronym for IS]. There are other radical movements," he said.
Since the Brussels attacks, an influx of tips has flowed into police departments and intelligence services, presenting a major challenge in classifying threats.
"There are many more alerts than before. We have received 600 reports a day," Jacobs said in the interview.
"A lot of the tips are not relevant but when we're told that some gray car is suspect, if we don't verify it and there is an attack the next day, they are going to say that we knew and didn't do anything," he said.
Some 45 percent of the Belgian judicial police were dedicated to terrorism cases, he said.
"In the later days, we found that in some things we lacked experience and called foreign police, for the analysis of explosives, for example, to process certain information quickly," he said.
"We are in a phase where we have to catch up. While we are waiting to improve capabilities, we have to set priorities," he said.
Since the March attack, Belgian police have conducted frequent raids on suspected extremists.
Last week they seized weapons and arrested a man in his 30s in the Brussels neighborhood of Schaerbeek.
In October, prosecutors charged four people with involvement in terrorist activities after police raided 15 homes in the cities of Ghent, Antwerp and Deinze.
In August, an attacker who pledged allegiance to IS attacked two police officers with a machete.
In June, a new suspect was arrested for involvement in the March attacks.
aw/kms (AFP, dpa, EFE, LUSA)