Police smashed the windows of a car and doused the driver with pepper spray during a dramatic arrest in Melbourne's CBD on Tuesday.

The woman was left screaming on the ground after being crash tackled by police on Russell Street in front of dozens of shocked onlookers.

A construction worker rushed to give police a bottle of water to pour over the woman's eyes as she continued to yell for help, while being pinned to the ground.

Officers first tried to speak to the woman about 10.30am after a member of the public told them the driver appeared to be unwell.