There are reports of at least one shooter at the Gilroy Garlic Festival and multiple casualties.
The shots were fired at the close of the three-day event at around 5.30pm local time.
It's currently unclear how many people have been injured and whether there are any fatalities, although some news outlets are reporting that up 11 people could have been caught in the gunfire with one person killed.
The gunman has not yet been apprehended.
Ambulance crews and police are on the scene.
A reunification centre has been sent up at a nearby school for families separated during the chaos following the shooting.
People attending the event have been sharing what they saw on Twitter.