David Mamet -- the guy directing the upcoming HBO flick about Phil Spector -- ignited a firestorm after publicly claiming Spector is innocent of murder ... and now the victim's friends are horrified that Phil "will be portrayed with some kind of sympathy."
Friends of late actress Lana Clarkson were FURIOUS this week when Mamet told the Financial Times, "I don't think [Spector] is guilty. I definitely think there is reasonable doubt ... they should never have sent him away ... If he'd just been a regular citizen, they never would have indicted him."
Now, Clarkson's friends have banded together -- and have fired off a letter to the director, claiming his comments were "mind-boggling and wrong in so many ways."
According to the letter, obtained by TMZ, the group's main concern is that "the loathsome, lying, gun-abusing convicted murderer of our friend Lana Clarkson will be portrayed with some kind of sympathy."
Lana's friends want to remind Mamet -- whose upcoming film stars Al Pacino and Bette Midler -- that Spector was CONVICTED by a jury of his peers ... and plead, "Please refrain from rewriting history for 'creative license.'"
Spector is currently serving 19 years to life in prison for murdering Lana in 2003.