Justin Bieber vs. Keyshawn Johnson -- Intense Showdown at Singer's Mansion

Keyshawn Johnson chased down a speeding Justin Bieber this weekend -- and tried to confront the singer about his dangerous driving habits -- but JB ran away like a scared little child ... eyewitnesses tell TMZ.

Sources tell us ... Keyshawn -- a former NFL superstar -- had just left a party Monday night inside of his exclusive gated community in Calabasas (with his child in the car), when Bieber zoomed by in his Ferrari at breakneck speeds.

We're told Keyshawn was furious -- feeling Justin could've killed someone -- so he brought his kid home, then chased after the singer in his Prius (yes, a Prius chasing a Ferrari).

When KJ got to Bieber's pad -- he blocked Justin's Ferrari in the driveway and got out of his car to confront Justin face-to-face ... but Justin ran inside of his house and refused to come out.

FYI -- Keyshawn is 6'4", 212 lbs ... Justin's about 5'7", 110 lbs (soaking wet).

As we previously reported, at least 2 people called the police on Justin that night -- and cops responded to the singer's home.

There's more ... sources say SEVERAL people witnessed the speeding incident -- and say Justin was definitely in the driver's seat. We're told the witnesses told Sheriff's deputies ... Bieber appeared stoned at the time.

We're still waiting to hear from Bieber's camp.

10:15AM PDT Now an NFL Hall of Famer is weighing in ... Eric Dickerson, who also lives in Calabasas, just tweeted ... "Justin Bieber needs to slow his ass down."