But prosecutors' spokesman Eric Van Der Sypt said the Belgian probe had been under way from before the January 7 attack on French satirical newspaper Charlie Hebdo.

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A third man was detained in the eastern city of Verviers, where police commandos ran into a hail of gunfire after trying to gain entry to an apartment above a town centre bakery. All three were citizens of Belgium, which has one of the biggest concentrations of European Islamists fighting in Syria.

Other raids on the homes of men returned from the civil war there were conducted across the country, said Mr Van Der Sypt, adding that they were suspected of planning attacks on Belgian police stations.

"The searches were carried out as part of an investigation into an operational cell, some of whose members had returned from Syria," he said. "For the time being, there is no connection with the attacks in Paris."

Describing events in the quiet provincial town just after dark, he said: "The suspects immediately and for several minutes opened fire with military weaponry and handguns on the special units of the federal police before they were neutralised."

Earlier, prosecutors said they had detained a man in southern Belgium who they suspected of supplying weaponry to Amedy Coulibaly, the killer of four people at a Paris Jewish grocer's after the Charlie Hebdo attack.

After the violence in Verviers, La Meuse newspaper quoted an unidentified police officer as saying: "We've averted a Belgian Charlie Hebdo."

Two French brothers, who like Coulibaly claimed allegiance to Islamist militants in the Middle East, killed 12 people at the offices of Charlie Hebdo.

Belgium has seen significant radical Islamist activity among its Muslim population.

Public television RTBF showed video of a building at night lit up by flames with the sound of shots being fired.

Marie-Laure from Verviers told RTBF she was in the street with her children when a police commando told them to run for cover.

"When we began running, we heard three or four big explosions and shots," she said.

Per head of population, Belgium is the European country from where the highest number of citizens have taken part in fighting with Syrian rebels in the past four years, data compiled by security researchers have shown.

Belgium has taken a lead in EU efforts to counter the threat perceived from the return of "foreign fighters" from Syria.

The Belgian government believes about 100 of its nationals have come back from there, while a further 40 may have been killed and about 170 are still in the ranks of fighters in Syria and Iraq.

hnews@herald.ie