An elderly man has been assaulted in Mandurah, receiving a blow to the back of his head that left him unconscious, after multiple offenders ransacked his house in the early hours of Saturday morning.

Mandurah detectives are investigating the serious aggravated burglary and assault that occurred on Denham Street in Dudley Park between 2am and 3am.

The victim, a man in his 80s, was woken by a group searching through various rooms in his home.

Upon getting out of bed to investigate, he received a blow to the back of the head causing him to lose consciousness.

The victim regained consciousness some time later to find his home had been ransacked. He was later conveyed to hospital for treatment of injuries, and is in a serious but stable condition.

Anyone with information, CCTV or dashcam footage of the area at the time of the incident, is asked to contact Crime Stoppers on 1800 333 000 or report the information online at crimestopperswa.com.au.

Callers may remain anonymous if they wish.

More to come.