Two people, the car's driver and a traffic policeman, are reported have been seriously injured. The accident took place near the Andheri Goldspot Junction in Mumbai's western suburb.
The truck, fitted with the very heavy cement mixer, was reportedly driving at great speed on the western express highway towards the airport.
The driver lost control on the flyover and the truck plunged and fell on the Innova car driving on a road below the flyover.
There were four people travelling in the car and the driver is said to be in a serious condition
A traffic constable has also been badly injured.
Traffic on the road has been diverted and the road had closed to clear debris
Two fire tenders, rescue vans and ambulances are on the site.
More details are awaited.