Convicted sex offender Brock Turner, whose six-month prison sentence for attacking an unconscious student at a party sparked international outrage, has been released after serving half his sentence.

Turner, aged 21, was handed a large packet of hate mail upon leaving Santa Clara County jail in the early hours of Friday morning, according to county Sheriff Laurie Smith.

Turner was jailed for his assault on a young woman near a rubbish bin after drinking at a Stanford University fraternity party. He was caught by two passing cyclists, who noticed the woman was not moving and chased him down.

His case hit headlines around the world after his victim read a poignant statement out in court, detailing the emotional, as well as physical, harm he had caused her.

Critics slammed Judge Aaron Persky for his "lenient" sentencing, during which he had lamented how prison could have a "severe impact" on Turner, and cited the "extraordinary circumstances" of his youth, clean criminal record and other considerations.

The outcry prompted California lawmakers to push for a tougher sexual assault law, and a social media campaign calling for Persky to be recalled which attracted more than 1.3 million signatures.

Persky has now voluntarily removed himself from hearing criminal cases, starting next week.

Turner's father Dan also sparked anger for his letter to the judge, in which he referred to the assault as "20 minutes of action out of his 20-plus years of life."

A one-time Olympic hopeful swimmer, Turner plans to move back to Ohio to live with his parents.

He faces three years of supervised probation and has five days to register as a sex offender, and will stay on the register for life.

That means he will be banned from entering parks, schools, and other places where children are expected to gather.

Over the three years, deputies will carry out unannounced checks on him to ensure he has not moved address without express permission from the authorities - and Greene County Sheriff Gene Fischer, who will be overseeing Turner's probation in Ohio, said he must also avoid alcohol and drugs during period.

Turner's neighbours will be sent postcards warning them that a convicted sex offender was moving in nearby, Fischer added.