Belgium called for the creation of a European intelligence agency Monday to counter the threat of jihadist violence in the wake of the Paris attacks.

Prime Minister Charles Michel told French radio RTL that "we must quickly put in place a European intelligence agency, a European CIA" to collate information on suspected radicals and "unmask those with hostile intentions."

A Belgian judge on Friday charged a sixth suspect with terrorism offenses in connection with the Paris attacks, prosecutors said, as the country pushes on with its investigation into links to the November 13 atrocities.

"Yesterday a person has been arrested in Brussels for interrogation. The investigating judge has remanded him in custody and charged him with terrorist murders and participation in the activities of a terrorist organization," the federal prosecutor said in a statement.

Belgium reduced the terror alert in Brussels Thursday, five days after it was raised to the highest possible level that saw schools and the metro closed, the government's crisis center said.

"We can confirm that the Threat Analysis Coordination Agency re-evaluated the threat level from four to level three," a spokesman for the crisis center, which is part of the interior ministry, told AFP.

Four of the main suspects in the Paris attacks were on a list of radicalized people compiled by Belgium's intelligence services as early as June this year, officials said on Thursday.

The list of 85 individuals included alleged Paris ringleader Abdelhamid Abaaoud, brothers Brahim and Salah Abdeslam and newly named suspect Mohamed Abrini, they said.

Belgian emergency services evacuated the biggest mosque in Brussels and decontaminated 11 people on Thursday after the discovery of a suspicious package containing what turned out to be flour, officials said.

With the EU capital on its highest terror alert, firefighters in chemical suits and gas masks were called in to the Great Mosque of Brussels amid fears that white powder in the package could be anthrax.

Brussels struggled to return to normal Wednesday after four days on maximum terror alert, with schools and the metro reopening despite two suspects from the Paris attacks still being at large.

Troops and police were still patrolling the streets of the Belgian capital and the alert status remained at the highest possible level of four, leading to questions about what had changed since schools had closed on Monday.

Belgian police arrested 16 people in a wave of anti-terrorism raids but key Paris attacks suspect Salah Abdeslam slipped the net, with Brussels set to remain under security lockdown on Monday.

Schools and the metro will be shut as Brussels stays at the highest possible alert level over what Prime Minister Charles Michel called a "serious and imminent" threat of attacks similar to those that killed 130 people in France.

Brussels will remain at the highest possible alert level on Monday with schools and metros closed over a "serious and imminent" security threat in the wake of the Paris attacks, the Belgian prime minister said, shortly before a major police operation got underway in the capital.

"Various operations are underway" in connection with the "terrorist threat," police said.

Belgian police arrested nine people in Brussels on Thursday during raids connected to last week's deadly Paris attacks, prosecutors said.

Seven people were "taken in for further investigation" during six raids linked to French national stadium bomber Bilal Hadfi, the federal prosecutor's office said in a statement.

Belgian Prime Minister Charles Michel on Thursday rejected criticism of his country's security services over the Paris attacks, saying Belgian intelligence led to a huge raid in France targeting the suspected mastermind.

"I do not accept the criticisms which were aimed at denigrating the work of our security services," Michel said in a speech to parliament in which he unveiled new security measures.