"We fear an attack like the attack in Paris ... Meaning several individuals conducting an attack in Brussels, possibly in several places at the same time."
The highly unusual measures in the Belgian capital underscore the fear gripping Europe a little more than a week after the co-ordinated assaults in Paris, which killed 130 people and injured more than 350 and brought into focus Isis' ability to strike the West directly.
Not since Boston came to a near-standstill after the Boston Marathon bombing in 2013 has the life of a major Western city been brought to a halt this way by the fear of terrorism.
The possibility that one or more of the Paris attackers may have arrived among the waves of refugees from the Middle East has intensified debate in the United States as well as Europe over their welcome in the West.
Belgian authorities say they have acted in response to a plot that included shopping centres and public transport as soft targets for a possible large-scale attack. The Government has warned residents to avoid crowded locations.
As raids were carried out, streets were shut down in central Brussels, and police urged residents not to post details of the operations on social media. Nineteen raids were carried out in the Brussels area and three more in Charleroi, about 80km south.
The Grand Place, the central square of Brussels, was evacuated because of a police operation. Guests in nearby hotels were told to stay indoors.
Justice Minister Koen Geens apologised for the shutdown of the subway system, which continues as authorities extend their hunt for suspects.
"We don't want to paralyse Brussels economically, but we need time to arrange all this," Geens said.
Belgian officials are casting a wide net for those involved in the Paris attacks and others who may be preparing to strike in Belgium.
A primary target is Salah Abdeslam, a 26-year-old French national who is the subject of a manhunt across Europe.
Abdeslam, who was identified as one of at least nine suspected attackers, is believed to have made his way to Belgium after the attacks.
He was not among those arrested yesterday, Belgian prosecutors said. In recent days, police raids in Molenbeek, the largely immigrant area of Brussels that several of the Paris attackers called home, and in other neighbourhoods had uncovered explosives and weapons, but neither was recovered yesterday, prosecutors said.
During a raid near a snack bar in Molenbeek, however, police fired two shots after a car rushed at them. It escaped but was later stopped and the wounded driver was arrested.
Most of the identified attackers are French and Belgian nationals and it is believed they became radicalised in Europe. Some are known to have travelled to Syria to fight with Isis.
Belgian Interior Minister Jan Jambon told Flemish national television that authorities still sought members of the suspected militant cell linked to a Brussels plot or their accomplices.
"It involves several suspects, and that is why we have put in place such exceptional measures," Jambon said.
The Government faces a difficult task in managing what it sees as an acute threat while also minimising the disruption and anxiety caused by security measures.
"We are following the situation minute by minute," Jambon said.
"There is a real threat, but we are doing everything possible day and night to face up to this situation."
