MONTEVIDEO, Uruguay—Tiny Uruguay embarked on an ambitious social experiment Wednesday by becoming the first country to regulate and oversee the sale of marijuana, a policy that has enthralled pro-pot activists and smokers abroad but has lukewarm support at home.

Under tight restrictions, the only establishments licensed to offer marijuana are pharmacies, 16 of which began to sell here in the capital. Pot connoisseurs lined up and then gushed about both buying marijuana legally and the product’s quality.

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