A cyclist has been killed after she was hit by a bus while riding in Mt Lawley.

Police said the cyclist had been riding along Beaufort St when she was struck by the bus at 2.20pm on Wednesday.

It is understood a passenger aboard the bus administered CPR before St John Ambulance arrived but the rider could not be saved.

A St John Ambulance spokesman said paramedics attempted resuscitation at the scene and transported the patient to Royal Perth Hospital, arriving at 2.45pm.

Major Crash police have launched an investigation.

Police said road blocks were in place and warned that there could be some congestion during the afternoon peak as police examined the scene.