Smoking in Sao Paulo will only be allowed at home or in the open air
A smoking ban in bars, restaurants and offices across the Brazilian state of Sao Paulo came into effect on Friday.
The ban outlaws smoking everywhere but in the open air and in private homes.
Sao Paulo governor Jose Serra says most people support the ban and he has pledged that anyone flouting the new law will be fined.
State officials said that about a dozen restaurants and bars had been fined in the first hours of the ban, AP news agency reported.
Sao Paulo state has an estimated 40 million inhabitants with about 11 million living in Sao Paulo city.
Some bar and restaurant owners have complained that the law is confusing and they say it will be difficult to enforce.
"How can I control someone who comes in and smokes inside?" asked Max Abdo, owner of the Tabaceria Ranieri in Sao Paulo city.
"I will respect the law, but everything is very confusing," he said.
Mr Abdo's business partner, Cassio Gabus Mendes, said that smoking a Cuban cigar "is a ritual" in their bar.
But he added: "Smokers are going to have to adapt."
Correspondents say that previous attempts to ban smoking in Sao Paulo have failed, largely due to public apathy and legal challenges.
This time, however, officials say they have the support of the vast majority of the population.
In an interview with Brazil's Globo TV, Mr Serra said the ban was to protect non-smokers, seven of whom die in the state every day from passive smoking.
Establishments which allow customers to flout the ban will be fined up to $750 (£450) for the first offence, with the fine doubling the second time.
For a third breach of the new law, the business will be forced to close for 48 hours and for a fourth offence, 30 days.