A truck with Bulgarian license plates overturned on the Athens-Lamia national highway, near the junction for Thermopyles, early on Monday.
It was not known what caused the truck, which was carrying laundry detergent, to flip over at 7.15 a.m. as it headed south.
There were no reports of injuries.
Part of the highway was shut down as authorities brought in cranes to move the heavy vehicle. Motorists have been warned to expect delays to their journey times.