According the LA Times:
According to his police report, Sgt. James Crowley said the professor was "yelling very loud" and "accusing me of being a racist."
Complaining that the "acoustics of the kitchen" made it difficult to communicate, the officer said he "told Gates that I would speak with him outside."
Once on the front porch, the officer arrested Gates for being loud and abusive in the presence of several neighbors who had gathered on the sidewalk.>