Belgian security forces killed two terror suspects in a fierce shoot-out in the eastern city of Verviers foiling a major attack against police buildings.
A third person was arrested in the raid, authorities said.
The suspects immediately opened fire on police when they closed in on them near the city's train station, Magistrate Eric Van der Sypt said in Brussels.
He said there was no link at this stage to the Paris attacks, and that the raid is the result of an investigation that has been under way for a few weeks.
"These were extremely well-armed men," with automatic weapons, Mr Van der Sypt said. Police buildings were the target, and an attack had been expected in hours or days, he said.
There was an intense firefight for several minutes on an upper level of a building in Verviers where the raid took place. The building is multi-storey and appears to be residential.
"We still expect a number of arrests," said Mr Van der Sypt. No police were wounded or killed in the firefight, which occurred at the height of rush hour in a crowded area.
The magistrate said more anti-terrorist raids were under way in the Brussels region and Verviers, adding that Belgium's terror alert level was raised to its second-highest level. The operation was part of an investigation into extremists returning from Syria, authorities said.
Verviers is a former industrial town with a large immigrant population about (80 miles southeast of the capital, Brussels.
Witnesses speaking on Belgium's RTBF radio described a series of explosions followed by rapid fire at the centre of Verviers, near a bakery and in the area of the train station. Video posted online of what appeared to be the raid showed a dark view of a building amid blasts, gunshots and sirens, and a fire with smoke billowing up.
Earlier today, Belgian 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 prosecutors say killed four people in a Paris kosher supermarket last week.
The man arrested in Belgium "claims that he wanted to buy a car from the wife of Coulibaly," said Mr Van der Sypt. "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 regarding the car, but he was arrested following a search on his premises when enough indications of illegal weapons trade were found.
Mr Van der Sypt stressed there was no established weapons link with the Paris attack at this moment.
Several countries are now involved in the hunt for possible accomplices to Coulibaly and the two other gunmen in the French attacks.