'Transformers 4' Director Michael Bay -- Air Conditioner Assailant Was an Extortionist!

Michael Bay just told TMZ ... the man who attacked him with an air conditioner was an extortionist who vowed to torpedo "Transformers 4" unless Bay paid him off.

Bay told us on "TMZ Live" that he was shooting in a Hong Kong neighborhood earlier in the week and had paid various businesses on the surrounding streets money in return for shutting down during the shoot. One dude held out, and, according to Bay, set out to ruin the shoot by turning on an electric saw and making other noises when the cameras were rolling.

Bay says the guy -- who he's convinced was on drugs -- came at him with the air conditioning unit.

The way he tells it ... very entertaining.