Bolivian police have confiscated two tonnes of uranium that was being stored at a building in central La Paz located near the US and Spanish embassies.
The radioactive material was in sacks of jute and nylon, he said.
Since Bolivia does not produce uranium, Perez said, authorities assume the consignment originated in either of two neighbouring countries that do: Brazil or Chile.
The commander of the elite police unit that carried out the operation, Col. Eddy Torrez, said the seizure was the fruit of a six-week investigation.
Police pounced when they learned the people in possession of the uranium planned to meet Tuesday with a potential buyer, the colonel said.
Perez said one of the people arrested is an engineer who told police he was holding the uranium for other people, but provided no information on the owners of the cache.