Europe on High Alert After Terror Sweeps

Thousands of officers are deployed.

— -- Nations across Europe remained on alert Saturday, deploying thousands of officers following terror sweepe by law enforcement agencies in the wake of last week’s deadly attack in Paris. In Greece, four suspected terrorists have been arrested, news agencies reported.

Up to 300 soldiers will be deployed starting today in the streets of Brussels and Antwerp, and possibly other areas, a spokesman for the interior ministry confirmed. A statement after a late night meeting Friday says their role is to watch various sites and back up the police.

Brussels police confirmed to ABC News that 14 local police stations would be closed during the night, which is thought to be a precautionary measure in the event of an attack. Belgian policed carrying out 12 raids Thursday afternoon and evening because attacks against police officers were planned for Friday, according to EU counter-terrorism coordinator Gilles de Kerchove. Belgian authorities have described the foiled plot as “very imminent.”

In Greece, a police source told the Associated Press that they have detained four suspected terrorists, including one who could be the man wanted by Belgian authorities as an alleged ringleader of a jihadi cell. The source spoke on condition of anonymity because the person was not officially authorized to comment on an ongoing investigation.

The person told the AP that the four were arrested Saturday, including one who "matches the description" of Abdelhamid Abaaoud -- the man who Belgian authorities suspect was the mastermind behind a dismantled jihadi cell there. Greek police have sent photos, fingerprints and DNA material to Belgium and are waiting to see whether the person is indeed Abaaoud, the source said.

In the UK, after attacks on police in Europe in recent days, officials raised the threat level for police officers across Britain to severe – the highest level it has ever been. It means an attack is “highly likely.” Police in London say they also have "heightened concern” about the risk to the Jewish community in the UK.

“In addition to our existing security measures, we are in dialogue with Jewish Community leaders about further actions that we will be taking, including more patrols in key areas,” said Mark Rowley, assistant police commissioner. “We have to be incredibly vigilant and look at all of these risks, particularly risks to police officers themselves and take every action that we can.”

The head of Europol, Rob Wainwright, told the BBC that the need for tightened security across Europe highlighted the complex nature of the terrorist threat in the region. "We're dealing with multiple thousands of potential terrorists," he told the BBC World Service. He said it was hard for police to identify plans because suspects were "working in a self-radicalised way very often, not necessarily under any command and control structure."

France remains on its highest terrorism alert level and authorities have said that some 120,000 police and soldiers have been mobilized across France.

One of Europe’s top counter-terrorism officials said yesterday that it’s impossible to guarantee 100 percent security and that other attacks could follow last week’s incidents in Paris.

“We know too much how prisons are major incubators of [extremism],” he said.

In the case of the Paris attacks, the gunmen in each assault had spent time in prison together and with other radical figures, an investigation after the attack found.

Rather, Kerchove said disengagement or rehab-like programs could be the answer.

“We will tackle the problem in a significant way, but it is a challenge,” he said.

ABC News' Lee Ferran contributed to this report.