A bus packed with Moroccan farm workers crashed through the security rails of an overpass in southern France and plunged onto a major highway Thursday, killing at least a dozen people.

The Operational Center for Fires and Rescues said 12 passengers died and 30 others were injured, several of them seriously.

All the victims were Moroccan.

Rescue squads struggled for more than four hours to extricate passengers from the mangled wreckage of the midmorning accident at a highway interchange north of this Mediterranean port. The bus crashed through security rails of the overpass near Coudoux and plunged 50 feet onto the A7 highway below.

Witnesses said the right front tire of the bus blew out as it climbed up the ramp of the overpass, sending the vehicle out of control.