He walked right out of jail and straight into a storm of protest.

Stanford rapist Brock Turner left jail on Friday, serving just half of his six-month jail sentence for sexually assaulting an unconscious woman.

His sentence and subsequent release sparked global outrage and the attention of protesters, some of whom were armed, outside his parents' Ohio home.

Carrying signs such as "castrate rapists" and "the rapist who lives here got three months, justice?" the group were waiting for the former Stanford university swimmer.

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However he was nowhere to be found, American broadcaster ABC reported.

It is believed Turner stayed at a hotel with his mother in Mountain View, California, on Friday night and it wasn't clear where he was for the rest of the weekend.

That didn't appear to bother the protesters from Dayton, Ohio, who scrawled obscenities on the driveway, WCPO-TV reported.

One told the broadcaster he wasn't going to live a happy or pleasant life.

"We're going to never let him forget what he did," the protester said.

Another revealed: "If he is uncomfortable then he begins to receive at least some punishment that he deserves for his crime."

Turner was convicted of assaulting a young woman near a garbage bin after they drank heavily at a party in January 2015.

The 21-year-old served his sentence at the Santa Clara County jail.

Following Turner's release from jail, Sheriff Laurie Smith said she believed his sentence was too light. "He should be in prison right now, but he's not in our custody," she told reporters.

Smith said jail guards gave Turner a big package of hate mail sent to him over the last three months and that Turner lived in protective custody in jail after receiving threats.

California jail inmates with good behavior typically serve half their sentences. Ohio prison officials earlier this month agreed to take over supervision of Turner's probation.

Greene County Sheriff Gene Fischer said Turner has five days to register as a sex offender with his office in Xenia, Ohio, 24km east of Dayton. He will have to report to a probation officer for three years and must avoid alcohol and drugs during that time.

Fischer said his department will notify Turner's neighbours informing them that a convicted sex offender is moving nearby. Turner will be required to register every three months in person at the sheriff's office, reaffirming that he is still living with his parents, the sheriff said.

Deputies also will check on Turner without warning to ensure he has not moved without permission from authorities.

- AP