Call it a big pot seizure at the big top: Mexican authorities found four tonnes of marijuana hidden under a circus truck.

Federal police discovered the drugs during a routine traffic stop on a road in the northern state of Sonora, which borders the United States, the national security commission said in a statement on Tuesday.

After smelling a strong odor of marijuana, the officers found the drug hidden in a secret compartment attached to the underside of a cage-like trailer carrying two motorcycles, the statement said.

Police found 389 aluminum and plastic bags of marijuana and released a picture of the massive haul lined up in front the red truck, which had the word "circo" (circus) on the side in yellow.

The pot was apparently headed to Tijuana, a northwestern city lying along the border with California. The driver and another person were detained.