Prosecutors kicked things off Wednesday in front of a jury, saying ... "The defendant, Robert Kelly, had sex with multiple children. He made videotapes of himself having sex with children," while also pointing at 2 co-defendants, ex-associates of Kelly's, whom they claim knew about it while it was going on and allegedly helped keep things quiet.

The woman at the center of the '08 case -- who was alleged to have been videotaped in sexual encounters with Kelly, at the age of 14 -- was mentioned too. She was referred to by prosecutors as "Jane" ... and they say she's going to testify in this case about what happened with RK.

According to prosecutors, Kelly went to incredible lengths to buy Jane and her family's silence at the time ... allegedly shipping them out of the country on his dime, and having his employees move mountains to conceal her identity or any connection to some tapes.

There are 4 tapes in play during this trial, and prosecutors say they'll have the jury watch three of them. At least one of those tapes appears to pertain directly to the alleged '08 victim, and the remaining ones might have to do with other alleged victims from over the years.

Prosecutors have accused Kelly of having sex with at least 4 minors, as well as rigging the '08 trial ... in which he was acquitted. Kelly's attorney, however, fired back.

Jennifer Bonjean, who's now repping Kelly, said her client was the victim of a mob mentality prosecution -- one that she says is way too late, and clearly intended to paint him as a monster, even though he's already been through this wringer once before.

She called his accusers "criminals" who have tried to extort Kelly, and who actually have immunity deals with these same prosecutors. Bonjean also said that one tape prosecutors have alluded to, but won't play, doesn't exist -- and leaned on the fact that the Jane Doe (who's the '08 alleged victim) has long maintained she is not on tape with Kelly.