Pedestrians have described their shock as dozens of officers descended on Barrack Street this afternoon.
As many as six police vehicles were stationed in and around the Criminal Justice Service building shortly after 1pm.
An eyewitness said as many as 20 officers ran into premises and were also seen in the surrounding side streets.
A spokeswoman for the force said they were searching a number of people who were in and around the building on the corner of South Ward Road and Barrack Street.
One eyewitness said: “There were 20 officers who piled out of the cars; it didn’t look like anyone was arrested but the police seemed to be coming from everywhere.
“No one was arrested from what I could see but it must have been fairly serious given the volume of police.”