Uruguay has become the first country in South America to ban smoking in enclosed public places.
Thousands of Uruguayans die every year from smoking-related diseases
The ban came into effect at midnight local time (0300 GMT).
Now bars, restaurants or offices where people are caught smoking will face fines of more than $1,100 (£630) or a three-day closure.
The ban was decreed by President Tabare Vazquez, who is a cancer specialist. Cuba adopted a similar ban in February last year.
Mr Vazquez said the law would prevent unnecessary deaths from passive smoking.
An estimated 5,000 people die every year in Uruguay from smoking-related diseases, according to official figures.
President Vazquez said: "Passive smoking is also linked to chronic disease and premature deaths. Since there's not a secure level of exposition, the best thing to do is to ban cigarettes in enclosed places."
To promote the legislation, the government has launched a campaign called A Million Thanks, a reference to the number of Uruguayan smokers who will have to change their habits.
An opinion poll commissioned by the health ministry shows that 70% of the smokers support the ban.