Diddy arrested

                                

Diddy went to the campus yesterday to speak with Alosi -- believing he has an "issue" with Justin -- but things didn't go well when he arrived to campus.

the coach would not see him and staff told him to leave ... but Diddy went to Alosi's office instead, Alosi told him to get out and threatened to call security if he didn't leave. Diddy said, "Fine, I'll call police" ... and Alosi snapped -- going after Diddy with his hands. 

source said Diddy grabbed a kettlebell and held it up in a defensive position ... but did not attack with an intent to injure the coach. 

Diddy confronted Strength and Conditioning Coach Sal Alosi while Alosi was on the phone. We're told Diddy was told to "hang on" but he wasn't having it and picked up a kettlebell and swung it, narrowly missing Alosi. One source said if Diddy had connected, it could have killed Alosi.

Sources connected with Diddy said the rap mogul is adamant he acted in SELF DEFENSE.

P. Diddy has been arrested for allegedly getting into a fight with a football coach at UCLA ... where his son is on the team.

Diddy is still in custody at campus jail.