WESTPORT — A smoky fire Tuesday afternoon at the historic National Hall building downtown, which forced major thoroughfares to be closed during the afternoon commute, was quickly doused and traffic flowed again about 5:30 p.m.

Firefighters were dispatched shortly before 4 p.m. to the office building at 2 Post Road West as heavy smoke could be seen rising from the roof of the structure, which sits at the west end of the Ruth Steinkraus Cohen bridge downtown.

Shortly afterward as emergency operations unfolded, traffic was detoured around the Ruth Steinkraus Cohen / Post Road bridge over the Saugatuck River, the Route 33 / Riverside Avenue intersection and the Post Road West at Kings Highway North intersection.

Fire crews searched the top floor and attic space for the source of the blaze, Fire was discovered under the roof sheathing and was extinguished after access was made by firefighters on the roof and the top floor, according to the report.

Deputy Fire Chief Nicholas Marsan, interviewed at the scene, said that one firefighter suffered minor injuries, but was doing well afterward.

All occupants of offices in the building were evacuated safely, he said.

A large part of the building is under renovation, where the fire apparently ignited, Marsan said. While the amount of damage caused by the blaze was not immediately known, he said, it appeared most of the area being renovated did not suffer significant damage.

Assistance at the scene was provided by firefighters from Norwalk and Wilton, while crews from Fairfield and Weston provided backup coverage at the town’s fire stations.

Cause of the fire remains under investigation by the Fire Marshal’s office.

With additional reporting by Dawn Shmaruk.