Hamilton police investigated a possible “bomb threat” following a call out to city hall on Friday just after Noon.
In a Twitter post, the service said the building was evacuated for a search.
Traffic was rerouted for hours as a precaution near the building at Main Street between Bay and MacNab.
The roads were reopened just after 4 p.m.
As of late afternoon, investigators said they had completed a search of the building and gave the all clear around 4:30 p.m.
Detectives say a probe into the origin of the threat is still ongoing.