French police have released three of the 12 suspects arrested in an anti-terror sweep following the attacks in Paris by Islamic extremists that killed 17 people and put Europe on high alert.

Paris prosecutor spokesman Denis Fauriat said nine other suspects detained on Friday will have questioning prolonged by 48 hours, a step allowed under France's tough anti-terror laws.

Police in France, Germany and Belgium have arrested dozens of suspects in recent days as part of the anti-terror sweep sparked by the bloody spree in and around Paris, in which brothers Said and Cherif Kouachi and their friend Amedy Coulibaly killed 17 people at the satirical newspaper Charlie Hebdo, a kosher grocery and elsewhere.

Fallout from the attacks has spread around the world.

Demonstrations in support of the murdered Charlie Hebdo journalists have been held in countries from the United States to Brazil, and violent protests against the magazine's depictions of the Prophet Muhammad have taken place in Niger, Pakistan and Algeria.

French, German, Belgian and Irish police rounded up dozens of suspects earlier this week as anxiety spread across Europe and authorities rushed to thwart further attacks.

Belgian police launched a vast anti-terrorism sweep in and around Brussels and the eastern industrial city of Verviers on Thursday, which left two suspects dead. Police say the suspects were within hours of implementing a plan to kill police.

Yesterday, soldiers fanned out to guard possible terror targets in Belgium while police in Greece detained at least two suspects as part of the widening counter-terrorism dragnet across the continent.

Authorities say there was no apparent link between the foiled plots in Belgium and the terror attacks in Paris.

President Francois Hollande said France is "waging war" on terrorism, and it shows on the streets of Paris and elsewhere, where 122,000 police and well-armed troops have been deployed to protect the country.