Bam Margera, skateboarder and star of MTV's "Jackass" series, was detained by New York City Police early yesterday morning outside a biker bar. He allegedly got into a punching match with a cab driver ... then kicked in his car door. According to an eyewitness, it all went down outside the famous Hogs & Heifers Saloon in NYC. He saw Bam and his friend attack the driver through the car window, so he flagged down cops. We're told Bam walked away ... but the officers caught up to him. Bam tells a different story. According the him, the cabbie started the whole thing by calling his lady friend a "slut." Bam says the girl spat at the driver, who in turn grabbed her hair. Bam says he then dove into the car and began punching on the guy. Bam acknowledges the cops showed up "but couldn't figure out who to arrest," so everyone walked away ... free to do it again.

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