The man, who has been named locally as Jackie Mahood, was shot several times by men who went to an office on the first floor of the premises and singled him out shortly after 5pm.
The gunmen entered and left the building by a rear door.
Mr Mahood was taken to hospital with wounds to the head and neck. Police took a security video away for examination. The RUC Assistant Chief Constable, Bill Stewart, called the shooting "senseless".
Responsibility was not immediately clear. The office is an area accessible to republican groups but there were also suspicions that the attack could have sprung from internal loyalist feuding and unrest.
Earlier this week loyalists thought to be associated with the UDA went on the rampage in the same area in response to the RUC's action in arresting a prominent figure during a murder investigation. Petrol bombings and hijackings took place during protests.
In a separate incident yesterday a man was found handcuffed on a towpath near Lisburn, Co Antrim, with gunshot wounds in both legs.
London and Dublin have agreed on transferring 20 more republican prisoners in British jails to complete their sentences in the Irish Republic. Dublin sources said the deal will start with three IRA prisoners whose transfers are already pending, followed by six more, once enabling legislation has been passed by the Dail before Christmas. The remainder will move in the New Year.Reuse content