The suspects, whose identities and nationalities were not given, are part of one of the main drug trafficking rings based on the island of Ibiza, the intended final destination for the drugs, police said.
They shipped the truck from the Spanish port of Bilbao to Argentina, where it was loaded with the drugs in false bottoms at a farm located some 60 miles from Buenos Aires during one of the stages of the race.
At the end of the Dakar Rally, held between Jan. 1 and 16, the truck was sent back to Bilbao, where it was seized by police Friday upon its arrival.
"The vehicle had been totally transformed to adapt it to its supposed participation in the competition as a support truck, with publicity and logos of the event painted on its side," the police statement said.
"The drug trafficking ring even provided the vehicle with all the necessary technical and logistical material, including the official uniform of the competition."
Aside from the 814 kilos of cocaine, police also seized 15,000 ecstasy tablets, 4.5 kilos of hashish, two guns and just under €47,000 ($64,000) in cash.
Police suspect that the group intended to sell the drugs on the Spanish island of Ibiza this summer during the peak tourist season.
The Mediterranean island is traditionally a magnet for thousands of British and German vacationers, who are drawn to its sandy beaches and vibrant nightlife. Ecstasy and cocaine are popular drugs among clubgoers.