A man has been arrested after allegedly decapitating his father at their home in Bankstown, in the south-west of Sydney.

Police have confirmed that the head of the 69-year-old victim was cut off in the attack.

They allege his 42-year-old son ran at him with a knife.

Police have taken the alleged attacker into custody. He has not been charged.

The mother is being treated for shock in hospital.

Acting Superintendent Glen Fitzgerald said it was an awful scene, and police and paramedics had been offered counselling.

"A 42-year-old man from Villawood has been detained and brought back to the police station and is currently assisting police with the enquiries," he said.

"The scene itself is quite disturbing. It's been a difficult morning for both the family and the police, and ambulance are currently looking after their welfare."

The victim was well known in the community after moving to Australia from Lebanon 40 years ago, and many of his relatives have gathered outside the house.

They told media that the younger man pushed his mother down the stairs and then lashed at his father with a knife when he tried to help her.

The brother of the alleged attacker was at the house at the time and called triple-0.

The victim's grandson, Abdul Elesh, says he is devastated.

"He was a wonderful man, a beautiful man, well-known in the community," Mr Elesh said.

"He was just a warm, welcoming person, very peaceful. He was a lovable person."

Police were called to the home on Stacey Street just after 6:00am (AEDT) after receiving reports of a domestic assault.

Homicide Squad detectives have been called in to help Bankstown officers in their investigation.

A crime scene was established, which caused major delays for peak hour traffic.