Elderly US woman killed saving friend from sex attacker

Fusako Petrus
Fusako Petrus (supplied)

An 86-year-old Californian woman has died trying to fight a man who was allegedly sexually assaulting her 61-year-old friend.

Fusako Petrus was walking with her friend, Arden Arcade, on Wednesday morning (local time) when they were attacked.

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They were on a running track near Highlands High School when 18-year-old Neven Glen Butler allegedly jumped out and attacked Ms Arcade.

Sacramento Sheriff's Sergeant Tony Turnbull says Ms Petrus tried to defend her friend, hitting him with a "small walking stick".

"She died trying to help her friend," he said.

Butler was later arrested after a separate attack on an elderly person that afternoon.

He was being held without bail on charges of murder, felony assault and elder abuse, local authorities said.