More than 50 seniors were trapped after the bus they were travelling was involved in a crash with a truck, West Australian police said.

A police spokesman said the truck and bus collided at an intersection in Forrestfield, in Perth's south-east, at 10.55am (AWST) today.

About 55 seniors on the bus were being assessed at the scene by ambulance officers because of their age.

"They were trapped in the bus for a short period but now they are free," the police spokeswoman said.