Halton Police are on scene at a home on Lakeshore Road near 4th Line in Oakville, where they say one person remains barricaded inside.
They say the situation is contained to the home but are asking residents nearby to shelter in place.
Crisis negotiators are at the scene.
Police say they initially received a call reporting possible gunfire around 1:20 p.m.
Initially, two people were barricaded in the home but police say one person was safely removed.