A source close to Pratt tells us Stephanie had just left Holly Montag’s birthday party when she was pulled over. Our source says cops watched Stephanie get in her car and were waiting to bust her.

TMZ has learned Stephanie Pratt was arrested for suspicion of driving under the influence early this morning in Hollywood.

Law enforcement tells us Pratt, who appears frequently on MTV’s “The Hills,” was busted this morning at 3:45 AM PT in Hollywood.

She was taken into custody. Her bail is set at $5,000.

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