Bystanders fought for more than half an hour to save a man his 20s, who died in a suspected drowning at Lake Leschenaultia this afternoon.

Dozens of children were among those watching at the popular spot east of the city as the young man was dragged unconscious from the water to the shore, where bystanders started CPR.

A defibrillator from the lake cafe was used on the man and paramedics took over CPR after arrival but he could not be revived.

The man, said to be a strong swimmer, had been at the Chidlow lake with a carer.

Emergency services responded to a triple-0 call at the lake, just after 1pm.

Once a railway dam, Lake Leschenaultia became a recreational lake in the late 1940s and is popular with families and campers.

Today’s 37-degree temperature and public holiday status was expected to lead to a busy time on beaches and at swimming spots.

Mundaring Police and the Coronial Investigation Squad are investigating the death.

Police are seeking witnesses to the drowning and have asked that anyone with information call Crime Stoppers on 1800 333 000 or at www.crimestopperswa.com.au.