There was a horrifying incident at a St. Patrick's Day celebration near the University of Pittsburgh ... when more than a dozen people were injured -- some seriously -- after the roof of a house they were standing on suddenly collapsed, causing them to plunge to the ground.

Check out video posted to X, which captured Friday's tragedy ... starting with a large crowd of partygoers having a blast on the streets crammed with homes in Oakland, Pennsylvania.

As the camera pans to the right, we see a bunch of people hanging out on the porch roof of the house before the structure gives out -- and everyone drops like a sack of potatoes. It's unclear what caused the collapse.

Sixteen people suffered injuries and were taken to local hospitals for treatment. Three of the victims were in serious condition while 13 others sustained minor injuries.

Pittsburgh Public Information Officer Emily Bourne told CBS News there were hundreds of University of Pittsburgh students in the area at the time the roof crumbled.

Thankfully, no one harmed in the incident died -- as Officer Bourne stated students should be feeling lucky the outcome wasn't much worse.