A 73-year-old Latino man suffering from the early stages of dementia has been shot dead by a US police officer.

Francisco Serna was unarmed and standing in a neighbour's driveway when police in Bakersfield, California received reports of a man brandishing a gun.

Sergeant Gary Carruesco told KBAK-TV and the Los Angeles Times that police arrived at about 12.30 am and when a witness pointed to Mr Serna, an officer fired several rounds and killed him.

The Kern County coroner said Francisco Serna was declared dead at the scene in Bakersfield at about 1.15am local time on Monday.

Bakersfield Police Department said the officer who shot Mr Serna had been placed on administrative leave while the incident was being investigated.

Sergeant Carruesco confirmed that no gun was found in a search of the scene.

Mr Serna's son Rogelio told the Times he was a retired grandfather who did not own a gun. He said he was suffering from delusions and other early signs of dementia.

According to Eyewitness News, family members said Mr Serna often went on walks when he had trouble sleeping to make himself tired before bed.