Soldiers have fanned out to guard possible terror targets across Belgium, including some buildings in the Jewish quarter of the port city of Antwerp.
It is the first time in 30 years that authorities have used troops to reinforce police in Belgium's cities, just days after anti-terror raids netted dozens of suspects across Western Europe.
Up to 300 soldiers will be mobilised in Brussels and Antwerp.
Belgium has increased its terror warning to level three, the second highest, following the anti-terror raids on Thursday that left two suspects dead.
The head of Europol, Rob Wainwright, said the need for tightened security across Europe highlighted the complex nature of the terrorist threat in the region.
In the UK, police have been warned to be on their guard for a terror attack against them. "We're dealing with multiple thousands of potential terrorists," said Wainwright.
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".
Thirteen people were 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 led to the discovery of four military-style weapons including Kalashnikov assault rifles.
Investigators also found several police uniforms during the raids, which could have allowed the suspects to pass themselves off as police officers to launch an attack.
The searches came after police 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.
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.
"It shows we have to be extremely careful," said Van der Sypt. 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 added.
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 fire fight 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, which took place 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.
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