Justin Bieber Incident Eyewitness Claims JUSTIN SPIT

3/28/2013 9:30 AM PDT

Justin Bieber Incident -- Eyewitness Says Justin Spit

Justin Bieber now claims he never spit at his neighbor in a fit of anger ... but an eyewitness begs to differ.

TMZ broke the story ... Bieber was allegedly driving his Ferrari at speeds approaching 100 mph in the gated community ... this according to multiple neighbors.  The guy next door went onto Justin's property to complain, and he says Justin told him, "Get the f**k out of here," and then he spit in the guy's face and said, "I'm gonna f**king kill you."  The neighbor filed a battery report.

Sources connected with Justin tell TMZ ... the singer insists he did NOT spit in the neighbor's face.

We've learned the house directly across the street from Justin's is under construction, and several landscapers were in the front, watching the confrontation.  We've learned AT LEAST one landscaper told the construction foreman he saw Justin spit in the neighbor's face. 

There are various reports circulating that the neighbor had a thing against Justin, but we've checked with multiple neighbors who tell us the guy has never had beef with the singer.  In fact, the 47-year-old father of 3 -- who has lived in the Calabasas gated community for 10 years -- went to the guard shack shortly after Justin moved in and told the guards how important it was to maintain his privacy.

Cops are continuing their investigation.