Terrified motorists ploughed into sheep which had plunged onto a busy highway after the truck which was carrying them crashed on an overpass in Melbourne last night.

Around 400 sheep were being transported by the truck when it overturned on the Metropolitan Ring Road above the Princes Freeway at Laverton North just before 9:00pm.

Dead and injured sheep then plunged over the side of the overpass and onto the freeway below.

Motorists described a horrific and chaotic scene as falling sheep hit speeding cars and more animals wandered around the highway.

"We have one car that's rolled four times, three other cars that are extensively damaged from livestock falling through their windscreens and a truck hanging off the edge of a bridge," an ambulance spokesman said.

"At this stage of an evening it was an unusual finding.

"We are very fortunate we don't have far more human casualties out of this event. We do however have a large number of casualties in the sheep fraternity."

'We ran straight into them'

Gruesome: Dead sheep lie in the wreck of the truck ( ABC TV )

Hannah Sidebottom says a sheep fell on the roof of her car.

"I saw them coming down like a mountain of, it looked like dirt but obviously wasn't," she said.

"There was nothing we could do, they were like literally on top of us, we just ran straight into them.

"We all screamed. I slammed on the brakes and we are so lucky. My car is an absolute write-off, but the sheep, I just feel so bad for them. It's horrific."

A number of animals had to be put down by council rangers.

The rangers are working to clean up the rest of the dead sheep this morning.