A truck has been involved in a crash with seven cars on the M4 in Sydney's west. 

Twenty-one people have been assessed by paramedics and a man freed after spending more than an hour trapped in his van in Prospect at the Light Horse interchange.

Luckily, no one is believed to have suffered significant injuries but two people were taken to Westmead Hospital and the man freed from the van was taken to Liverpool Hospital.

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NSW Police said the driver of the truck was taken to Blacktown Hospital for mandatory testing.

The crash happened at 4.40pm, just as many started heading home on their evening commutes, with more than 11km of traffic now at a standstill.

All westbound lanes of the M4 are closed and traffic is being diverted at the Reservoir Road off-ramp.

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Motorists are being urged to use the Great Western Highway instead.

Eastbound lanes have reopened but traffic is also heavy in that direction and there are queues banked up more than 5km from Eastern Creek.