A black California man was brutally beaten by police officers after a convenience store clerk claimed he had a gun — and the department has opened an internal affairs investigation into the violent arrest.

William Colbert stopped at the ampm market last month to buy an iced tea on his way home from a hospital, where he was treated for an allergic reaction to medication, reported the Merced Sun-Star.

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The 39-year-old Colbert, a welder and pipe-fitter, got into a disagreement with the clerk over payment and asked the employee to put the money back on his credit card.

The two argued for several minutes before the clerk called police and accused Colbert of having a gun — which he denies.

Colbert was standing outside the store when police arrived because the clerk had locked him out, and he recognized the first officer as a classmate from Merced College.

That officer, Joseph Opinski, went inside the store to speak with the clerk.

The next officer to arrive, who Colbert identified as William McComb, then charged at the black man “like a bat out of hell” and took him into custody.

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Colbert asked why he was being arrested if he hadn’t committed a crime, and he said at least two more officers “bum rushed” him and threw him face first into a puddle.

He tried lifting his face from the puddle to breathe, but said he was trying not to resist being placed into handcuffs because he was “afraid to fight for my life.”

Police placed him in the back of a patrol car, and Colbert admits to kicking the cruiser’s door before asking for the tight handcuffs to be loosened — and officers pulled him out and pressed their knees into his back.

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“There should be no reason I come out of that cop car,” Colbert said.

He asked for medical attention, and McComb suggested he knew Opinski because he was a “snitch bitch gang member” — which Colbert said made him “irate.”

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McComb stayed in the hospital room throughout Colbert’s examination, and he admits to calling police “cowards” and one officer a “b*tch” out of frustration.

He was taken to the Merced County Jail without handcuffs because he was wearing a splint on his thumb due to an injury from his arrest, and he and McComb were met there by a black officer he later identified as Brandon Wilkins.

Colbert said officers began “aggressively” shoving him behind during processing, and one officer stumbled as he stepped aside to avoid a shove — and Colbert said he went to the floor to avoid additional force.

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Police then began kicking and punching him from behind, and an officer raked his arm with handcuffs, Colbert said.

Colbert’s mugshot shows his face bloody and bruised, with one eye swollen shut.

He was charged with resisting arrest, threatening an officer and battery on an officer.

Merced police launched an internal investigation, but they declined to comment on the case or release surveillance video of the altercation at the jail.