Cops have denied that a stabbing took place at a midterm rally where President Donald Trump spoke ... and revealed that an officer's knife became accidentally caught in a MAGA supporter's shirt.

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In a video taken of the chaotic scene on Wednesday night ... a MAGA supporter sits down on the curb with what at first glance looks to be a pocket knife sticking out of his back.

While social media quickly speculated that a stabbing had taken place while Trump was on stage, the Chillicothe Police Department says that's not what went down at all.

Chillicothe Police Captain Michael Short tells TMZ ... "There was no stabbing. The incident involved a fight, which an officer intervened to break up. The retention clip securing the officer’s pocketknife became caught in one of the individuals’ shirts during the altercation. The pocketknife did not open."

The incident went down outside the Shoemaker Center on the Ohio University Chillicothe campus -- where Trump held his speech.

Trump's visit drew a small group of protesters ... the Scioto Valley Guardian reported.