Fifteen people have been detained for planning a terrorist attack on policemen in Belgium, authorities said Friday, while warning the public to remain vigilant.

“I think we gave an important blow to terrorism in Belgium,” prosecution spokesman Eric Van der Sypt said.

“[But] nobody can really predict what’s going to happen in the following hours or days,” he added. “I think it’s a normal reaction that you keep the level of threat high because you have to be prepared.”

Police on Thursday carried out 12 raids in six Belgian communities, after it became clear that a group they had been surveying was poised to commit a terrorist attack.

“It could have been hours, certainly no more than a day or few days,” Van der Sypt said.

The aim was to kill policemen in the streets and in police stations across Belgium. Several of the group’s members – most of which are Belgian – had spent time in Syria, but Van der Sypt declined to comment on any links they might have with radical groups.

Belgium is commonly thought to have the largest contingent of European-born foreign fighters in relation to population size.

Two suspects were killed in the eastern city of Verviers after they opened fire on investigators. There, police recovered four AK-47 rifles, four handguns, ammunition, explosives, police uniforms, fake documents and an “important sum of money.”

One person was detained in Verviers, while 12 others were taken into custody in three Brussels neighbourhoods. Two Belgian nationals were also held in France. All are suspected of participating in the activities of a radical organization, Van der Sypt said.

He declined to give any information on their identity or to comment on reports that some suspects may be of Chechen origin. He said those in custody included not only people who were going to carry out the attacks, but also individuals “responsible for the logistics.”

“We have arrested already the people we wanted to arrest, but I cannot confirm that we have arrested everybody from this group,” he noted.

The operation comes one week after attacks in France, but Belgian authorities say they have no indication that the two incidents are linked. Officials also said there appears to be no connection to the arrest of two suspects in Germany.

Prime Minister Charles Michel said he is not aware of any new threats against Belgium.

He called for EU leaders to hold formal talks in the wake of the incidents, the Belga news agency reported. The leaders are due to meet next on February 12, but only for an informal meeting.

The Belgian capital is home to many of the European Union’s institutions and the NATO military alliance. The European Commission has put in place extra security measures after Belgium raised its alert level from 2 to 3, spokeswoman Mina Andreeva said.

The Belgian government on Friday agreed on 12 measures to fight radicalization, many of which had already been in the pipeline before the foiled attack. Legislative proposals are expected to be approved by mid-February, Belga said.

The measures include stepping up the fight against radicalization in prisons, creating a new criminal offence for travelling abroad with terrorist aims, and introducing more reasons for the withdrawal of Belgian nationality, the temporary confiscation of identity cards and the freezing of assets.

Belgian authorities have also implemented immediate measures to ensure the safety of law enforcement.

Many police stations in Brussels and in other cities are no longer accessible to the public, with enhanced security measures in place. Policemen have reportedly been told not to be on patrol alone.

Jewish schools in the Belgian capital and in the northern city of Antwerp have also cancelled classes, according to media reports.

The previous attack seen in Belgium had targeted its Jewish Museum last year.

A Jewish school in the Dutch city of Amsterdam also remained closed on Friday over the incidents in Belgium, although its management wrote in a letter to parents that there was no concrete threat. Extra security measures for Amsterdam synagogues were also put in place. SAPA