Yonge Street is closed in Bradford West Gwillimbury from 12th to 14th Line for a collision, South Simcoe Police Service announced this evening through Twitter.
Police said they responded around 4:30 p.m. to a "serious" two-vehicle collision involving a commercial vehicle and an SUV.
The number of injured people was not yet known Friday evening, but news reports indicate at least one person was transported to Southlake Regional Health Centre in Newmarket with life-threatening injuries.
The road was closed in both directions as crash reconstruction officers investigate at the scene.
Shortly after 6 p.m., police said Yonge Street would remain closed for "several hours."
Police recommend people avoid the area and find alternate routes.