It went down Sunday night ... Arnold tweeted, "Mark Burnett just went apeshit & choked me at this huge Emmy party then he ran away with his torn pink shirt & missing gold chain. I'm waiting for LAPD."
Burnett's wife, Roma Downey, says Arnold "tried to ambush" her and Mark. She says Arnold hurt her and she posted a pic of a bruise on her hand, claiming Arnold was the culprit.
Arnold, who said he was waiting for the LAPD, fired back, "Bullshit. You lie your crazy husband attacked me you psycho. I'm filing a police report & suing you for defamation."
We checked with LAPD ... so far no report.
Right after the fight, Tom posed for pics with other people in the party, including Patton Oswalt and Billy Eichner ... who cheekily said, "Everyone in LA is shaken. Hold your loved ones tight."
Arnold has been working on a show for Vice ... a show about so-called "hidden Trump tapes." One of the things Arnold is after ... the outtakes from "The Apprentice," which Burnett created and produces. Arnold believes the tapes reveal Trump using the n-word and other epithets. Burnett has said he has no access to the tapes because they're owned by MGM.