Local worker Paul Scotting said the scene was "like a plane crash".
He said it appeared the fully laden truck and trailer had hit a van doing a U-turn on the busy arterial road.
The truck and trailer then lost control and smashed into another truck unloading a Porsche at the luxury car dealership, Mr Scotting said.
That Porsche had then tipped off the truck into another car at the yard, he said.
"It's carnage," he said.
Mr Scotting said the van involved in the initial collision had been shunted across the road into the side of a parked BMW.
Two more cars, including another BMW, had also been damaged in the aftermath of that collision, he said.
About 10 police cars along with four fire engines and multiple ambulances attended the scene.
Hundreds of people gathered near the Porsche dealership.
The crash caused major traffic delays.