A detective says Russell Brand is still a danger to women ... despite how charismatic his public persona may be.

Sky News asked Detective Superintendent Andy Furphy -- who oversaw the investigation in Brand's sexual assault case -- if he believes Brand still poses a risk to women and he said ... "Yes, I do."

He told the outlet ... "I think he's got that position of power and trust and millions of followers."

Furphy said the actor was initially "withdrawn and quiet" in police interviews, but eventually became "more open" once he got comfortable.

He also said Brand's image is dramatically different from who he is behind closed doors, explaining ... "Out on the steps he's got his public persona, he is greeting the press but in court a very withdrawn character, particularly so when he gave his evidence."

As TMZ reported ... Russell was convicted Thursday of one count of rape and three counts of sexual assault.

He was acquitted of two other rape charges and attempted indecent assault ... and will remain behind bars until his sentencing Monday.