PALO ALTO (AFP) - Prestigious Stanford University's commencement was overshadowed on Sunday (June 12) by a sex assault case that prompted a brief protest by students.

A handful of students carried signs that read "Rape is Rape", "It doesn't matter what she was drinking" or "Stanford protects rapists" during the so-called "Wacky Walk" processional into the university stadium.

The event kicked off with the university's president John Hennessy calling for a moment of silence for survivors of sexual assault as well as victims of the massacre in Florida that left 50 people dead.

"As events on our own campus and around the country remind us, violence in all forms has become a scourge on our society," he said.

At the ceremony itself, there was only muted protest with several students sporting on their caps a message that read "1/3", in reference to the ratio Stanford students say will be victims of sexual assault by the time they finish a four-year degree.

A plane hired by a women's activist group earlier flew overhead, trailing a banner that read, "Protect survivors. Not Rapists. #PerskyMustGo".

The tag referred to Santa Clara Superior Court Judge Aaron Persky, who earlier this month sentenced to six months in jail a former student - Brock Turner - who was convicted for the 2015 sexual assault of an unconscious woman.

The sentence, which gained attention after the victim's statement to the court was made public, has prompted widespread outrage and calls for Judge Persky to be recalled.

The judge has not commented on the case as Turner's attorney has indicated he may appeal his sentence.

Emmy Award-winning filmmaker Ken Burns, who spoke at the ceremony, urged the nearly 5,200 graduates from 35 countries to be vigilant about sexual assault and to speak out against it.

"I am the father of four daughters," he said, addressing the students.

"If someone tells you they have been sexually assaulted, take it effing seriously and listen to them," he added to sustained applause.