It was reported shortly after 4 when a 2 inch gas main has been ruptured during water repairs.
Nearby residents along Crown Street and part of King to Pitt Street are being evacuated. At least three City Transit buses are being brought in for the evacuation.
Traffic in one of the busiest areas of the city has been blocked off and trains that would make their way along the adjacent rail line are also being stopped.
Police, Fire, Saint John Energy and Enbridge Gas have all been called in.