7:30 a.m. Update: JFRD crews have cleared the scene and all lanes have reopened.
6:09 a.m. Update: All lanes of West Union Street between North Laura Street and North Main Street are closed, according to the Jacksonville Sheriff's Office.
When crews arrived, fire was seen coming from a second-story window of a commercial building located at West Union Street and Main Street, officials said.
The Jacksonville Fire and Rescue Department responded to the scene of a fire in Downtown Jacksonville early Monday morning.
Photos: Jacksonville firefighters investigating after fire at building on Main Street
The fire is now under control, JFRD said.
Firefighters tell Action News Jax reporter Elizabeth Pace the building has been vacant for years.
JFRD is working to learn what caused the fire.