The International Narcotics Control Board (INCB) warned that the move, which includes production, sale and consumption of marijuana, would endanger young people and could lead to addiction.
The new law will allow registered Uruguayans over 18 to buy up to 40g of the drug a month, the BBC reports.
INCB chief Raymond Yans said he was 'surprised' that the government in Montevideo had 'knowingly decided to break the universally agreed and internationally endorsed treaty'.
The INCB is an independent body of experts established by the United Nations to monitor countries' compliance with international drug treaties.
After nearly 12 hours of debate on Tuesday, senators gave the government-sponsored bill their final approval.
The move has made Uruguay the first country in the world to legalise the production and sale of marijuana.
--ANI (Posted on 13-12-2013)