Kellen Winslow Jr. threatened to murder BOTH of the women he raped if they spoke to anyone about the incidents ... this according to prosecutors.
The ex-NFL star just appeared in court -- in shackles -- where officials spelled out the case against him.
Prosecutors say Winslow picked up 2 hitchhiking transients -- one in March and one in May -- drove them to an isolated location and raped them.
In both cases, officials say Winslow threatened to kill them if they told anyone.
As for the alleged attempted rapes of two elderly women -- 71 and 86 -- officials say in both cases he broke into their mobile home units with the intent of raping them ... but ultimately left before anything happened.
Prosecutors say cops have had Winslow under surveillance for a while -- and saw him trying to sell his H2 Hummer in an attempt to destroy evidence.
The judge ultimately deemed him a danger to the public and ordered him to be held without bail.
Winslow pled NOT GUILTY to all of the charges against him.
Inside the courtroom, Winslow was supported by family and friends ... including his father, Kellen Winslow Sr. -- the former NFL star.
As we previously reported, Winslow was arrested Thursday after officials say they found evidence to charge the ex-NFL star with 9 felonies ... including forcible rape, forcible sodomy, kidnapping and burglary. The 34-year-old has been in custody since then.
If convicted on all charges, the former Cleveland Browns tight end faces life in prison.