"At this point we are trying to confirm the sighting and locate the man. There has been no threats or reports of shots fired," the force tweeted.
Police received a call about a man wearing black cargo pants and an ear piece, concealing what was reported to be a rifle wrapped in black cloth at 8:36 a.m.
Officers are urging calm and asking people to follow their directions.
They are restricting access at Citadel High School and there's a heavy presence of police officers carrying assault rifles in downtown Halifax.
Halifax Regional School Board spokesman Doug Hadley said the high school is in a "hold and secure" mode. Students are not allowed to leave the building.
City Hall is now in lockdown. The public gallery at the legislature will be closed to the public today — that decision was made yesterday following the fatal shooting of a soldier guarding the National War Memorial in downtown Ottawa.
There are extra security people at the Halifax provincial courthouse on Spring Garden Road. The Capital District Health Authority has has also heightened security at the doors of the Halifax Infirmary.
Police are looking for any related information and possible witnesses. People are asked to call at (902) 490-5020.