Thirteen people have been detained in Belgium and two in France in an anti-terror sweep following a gunfight in which two suspected terrorists were killed.
Authorities said a dozen searches in the Belgian city of Verviers had led to the discovery of four military-style weapons including Kalashnikov assault rifles.
It comes after police yesterday moved in on a suspected terrorist hideout at a bakery in the eastern city, killing two suspects and wounding and arresting a third.
The arrested suspect was charged with belonging to a terrorist organisation.
Unlike the Paris terrorists, who attacked the office of a satirical newspaper and a kosher grocery store, the suspects in Belgium were reportedly aiming at hard targets: police installations.
Eric Van der Sypt, a federal magistrate, said the terrorists were aiming to kill police on the street or in their offices.
"As soon as they thought special forces were there, they opened fire," he said. "They were on the verge of committing important terror attacks."
Across Europe, anxiety has grown as the manhunt continues for potential accomplices of the three Paris terrorists, all of whom were shot dead by police. Authorities in Belgium signalled they were ready for more trouble by raising the national terror alert level from two to three, the second-highest.
"It shows we have to be extremely careful," Mr Van der Sypt said. The Verviers suspects "were extremely well-armed men" equipped with automatic weapons, he added.
Authorities have previously said 300 Belgian residents have gone to fight with extremist Islamic groups in Syria. It is unclear how many have returned.
Prime Minister Charles Michel said the increase in the threat level was "a choice for prudence".
"There is no concrete or specific knowledge of new elements of threat," he said.
The suspects in Verviers opened fire on police when officers closed in on them near the city's railway station, the magistrate said. There was an intense firefight for several minutes.
Video posted online showed a dark view of a building amid blasts, gunshots and sirens, and a fire with smoke billowing.
No police were wounded or killed in the clash at the height of the evening rush hour in a crowded neighbourhood of the former industrial city of 56,000, about 80 miles south east of the capital Brussels.
Earlier, authorities said they were looking into possible links between a man they arrested in the southern city of Charleroi for illegal trade in weapons and Amedy Coulibaly, who killed four people in a Paris market last week.
The man arrested in Belgium "claims that he wanted to buy a car from the wife of Coulibaly", Mr Van der Sypt said. "At this moment this is the only link between what happened in Paris."
At first, the man came to police himself claiming there had been contact with Coulibaly's common-law wife Hayat Boumeddiene regarding the car, but he was arrested following a search of his premises when indications of illegal weapons trading were found.
A Belgian connection figured in a 2010 French criminal investigation into a foiled terrorist plot in which Coulibaly was one of the convicted co-conspirators.
The plotters included a Brussels-area contact who was supposed to furnish weapons and ammunition, according to French judicial documents.
Belgian authorities did not give details of the people detained or killed, but said most were Belgian citizens.
Officials stressed that the targets of their crackdown had no known connections to last week’s attacks in France.
Mr Van der Sypt said: “I cannot confirm that we arrested everyone in this group.”
Belgium has seen a particularly large number of people join extremists in Syria, and is “the worst affected country in Europe relative to population size”, said Peter Neumann of the London-based International Centre for the Study of Radicalisation.
He estimates 450 people have left Belgium to fight with Islamic radical groups in Syria, and that 150 have returned home.