Mayor of Detroit -- PLEA for Mercy on Jalen Rose

Jalen Rose has some powerful friends -- including the Mayor of Detroit -- who wrote up a very personal letter to the judge in Jalen's DUI case to show the NBA star is remorseful and deserves mercy.

TMZ has obtained a copy of the letter written by Mayor Dave Bing -- a former Detroit Pistons star who is considered one of the greatest 50 NBA players of all time.

In the document, sent to the court before Jalen's sentence was handed out, Bing says, "I have known Jalen since the day he was born."

He adds, "I can assure you that Jalen fully comprehends the danger he posed on March 11. Jalen has assured me, but more importantly, he has assured himself, that this will never happen again."

Bing concludes, "I fully believe Jalen was truthful to me when he stated: 'I will never drink and drive again or ever be in any other court for any violation of the law."

Unfortunately for Jalen, Bing's letter -- and other letters written to the judge from friends of Rose -- weren't effective enough ... and the judge ordered him to serve 20 days in jail.