Few people appeared to be rushing to register with the government on the first day. Juan Vaz, a well-known cannabis activist, said he registered and found the process easy but can understand why some might be reluctant.
"There are some people who might feel persecuted," Vaz said. "For many years, they grew plants in secret and it's hard to break from that way of thinking."
Uruguay is the first in the world to attempt to regulate the cultivation and sale of marijuana on a nationwide scale.
The law, passed by Parliament in December 2013, also allows for the formation of growers and users clubs and the sale by pharmacies of 40 grams of pot a month to registered users.
So far, no club has yet completed all the requirements to begin operations but at least four have started the process. President Jose Mujica has said the sale through pharmacies will be postponed until next year.
Presidential and legislative elections are scheduled for October and the major opposition candidates have signaled that they intend to repeal all or part of the law if they gain the presidency or a majority in the parliament.
The leading opposition candidate, Luis Lacalle Pou, has said he would repeal the sale at pharmacies but allow the option to grow marijuana for personal use.
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