BEIJING — The loudspeakers in the pricey Temple bar erupted at 11:50 p.m. on Sunday with an announcement that set off either joy or despair, depending on the customer: Patrons had 10 more minutes to smoke.

A tough new ban on smoking in public took effect at midnight in the Chinese capital, aimed at ending the era of tobacco fumes clinging to clothes and lungs. The estimated four million smokers in the city — one-fifth of the population, the government reckons — now have to avoid lighting up in any enclosed public space, including offices, shops, bars, restaurants, nightclubs, airports and trains.

Some outdoor spaces are to be smoke-free now as well, including areas outside schools and hospitals as well as some tourist sites. Thinking of lighting up at the Forbidden City or on the Great Wall? Then be prepared for a fine of up to 200 renminbi, or about $32.

And if you are a business owner who permits smoking on your premises or fails to post signs and other information about the ban, including the phone number for reporting violations, the fine can run up to 10,000 renminbi ($1,600).

State news media cheered the ban as necessary to protect the public from dangerous secondhand smoke, but many Beijing residents were skeptical that it would actually be enforced. The municipal government has banned smoking twice before — in 1996 and before the 2008 Olympic Games — and each time the ban was widely ignored, with smoke continuing to waft through hotel lobbies, public lavatories and gym locker rooms.

City officials say this time will be different. Thousands of “health police officers” trained by the Beijing Health Inspection and Supervision Bureau will fan out to make sure the law is carried out, according to an official at the Beijing Municipal Commission of Health and Family Planning who gave only his surname, Ma.

“We are not depriving smokers of the right to smoke,” the official said by telephone, explaining that the law merely restricts where they do it. “I’m confident that it will be carried out smoothly and guide people to quit smoking.”

Suan Weiqing, a waitress at a dumpling restaurant, was less sure. It would be very difficult to stop diners from smoking, she said: “You can’t tell them they’re not allowed to smoke. We’re only a small restaurant, and we can’t offend our customers.”

With a cigarette in one hand and a pair of chopsticks in the other, Zhang Jiawei, a man in his 50s who said he has been smoking for more than 30 years, said that he supported the ban for health reasons and that he planned to smoke less because of it.

His friend Gao Jianjun, seated across from him, disagreed. “Cigarettes are a good thing,” he said. “I feel sick when I don’t smoke.”

Cigarettes are something of a national pastime in China, especially among men. Wedding banquet tables routinely feature plates of stacked cigarettes for guests. Cartons of expensive brands have long been given as presents or to curry favor with officials.

Smoking is also important to the country financially: A state-run monopoly manufactures one-third of the world’s cigarettes and accounts for a significant share of state revenue.

China has some 301 million smokers, almost one-third of the world’s total, according to the World Health Organization. Nearly 53 percent of men and 2.4 percent of women smoke regularly, the organization estimates, and the habit contributes to a million deaths a year from heart disease, cancer and other ailments. Secondhand smoke causes an additional 100,000 deaths a year, the organization estimates.

In Beijing alone, smokers burn through 14.6 million cigarettes a day on average, according to a study conducted last year by the Beijing Patriotic Health Campaign Committee. The study found that secondhand smoke was probably inhaled by 90 percent of people who went to bars and clubs, 65 percent of diners in restaurants and 40 percent of people in their own homes.

Against that backdrop, the ban’s impact could be significant. “Beijing has now set the bar very high,” said Dr. Bernhard Schwartländer, the W.H.O. representative in China, in a statement. “We now look forward to other cities around China, and the world, following Beijing’s excellent example.”

One literature teacher at a Beijing public high school recalled a faculty meeting last week when administrators reviewed the rules for the next year, including the new ban on smoking near the school. “A lot of the other teachers asked, ‘What are we going to do, because taking smoking breaks is so important to us?’ ” said the teacher, who asked not to be identified because she was not authorized to speak publicly.

Privately, however, the teacher was thrilled with the new ban. “Cigarettes are the devil’s instrument,” she said. “I wish they were banned from every Beijing street.”

If there is one industry that stands to lose from an enforced smoking ban, it is the city’s often extremely lucrative nightclubs. In many, the cost of a private table can start around 6,000 renminbi ($970), not including alcohol, and the prospect of wealthy customers staying home to engage in their favorite vices is a major concern.

As a result, many clubs have decided to ignore the ban and bet that the government will not really enforce it. “If we don’t let people smoke, they’ll stop coming,” said one club employee, who refused to be identified to avoid scrutiny from the authorities. Like many nocturnal establishments in the capital, the employee’s workplace has strong political connections to the district government, greased by another employee whose job “is just to drink with officials.”

The employee said those officials were expected to warn the nightclub before any inspections, so there would be ample time to extinguish cigarettes and clear away the ashtrays.

Beijing’s night life centers will quit the habit only if the authorities are serious about the smoking ban, the employee said. “We’ll just wait until the government makes an example of some club and hope it’s not ours.”

Sure enough, as the moon rose over Beijing on Monday, a number of establishments looked — and smelled — just as they had before the ban took effect. At a recently opened bar not far from the Forbidden City, patrons drank glasses of beer and smoked cigarettes in full view of the management, a customer said, adding that the bar “has very fancy ashtrays that they aren’t letting go to waste.”