Video posted to social media shows the moment an unleashed pit bull attacked a security guard at San Francisco’s Main Library Sunday.

A spokesperson for San Francisco Public Library said the incident unfolded after library patrons observed the dog barking next to its owner — who appeared to be unresponsive at a computer kiosk.

Given the common illegal drug use in the neighorhood, two security guards attempted to approach the owner to potentially administer Narcan, a medicine used to reverse overdoses from opioids — when the unleashed dog began “exhibiting aggressive behavior” and attacked one of the guards.

Two other guards tried to intervene and rescue their colleague.

Library officials said the owner was “largely unresponsive during the attack” until another patron woke him up.

Once finally awake, the owner managed to control his pit bull who he claimed is a service dog.

Animal Control took the canine into custody and the owner was issued a citation.

The security guard was taken to an area hospital where he was treated for serious injuries and is now at home recovering, according to library officials.