Minnie Driver went to the beach in Malibu with her son, but their day took a dark turn when a man allegedly started masturbating nearby ... and police got involved.
Law enforcement sources tell TMZ ... sheriff's deputies responded Sunday to Paradise Cove Beach Café just before noon for a report of a man pleasuring himself, and Minnie was one of the witnesses who told them the guy was cranking one out in a cabana.
Our sources say the alleged masturbator was gone by the time cops arrived ... and we've got video of Minnie and another woman speaking to deputies.
We're told Minnie and the other beachgoers didn't want to press charges, and simply wanted to make police aware of the situation.
As a result, our sources say the Los Angeles County Sheriff's Department beach team is aware and will be keeping an eye out ... just in case the guy gets another urge.