HANOI, Vietnam — As strips of tofu sizzle beside her in a vat of oil, Nguyen Thu Hong listens for police sirens.
Police raids on sidewalk vendors have escalated sharply in downtown Hanoi since March, she said, and officers fine her about $9, or two days’ earnings, for the crime of selling bun dau mam tom — vermicelli rice noodles with tofu and fermented shrimp paste — from a plastic table beside an empty storefront.
“Most Vietnamese live by what they do on the sidewalk, so you can’t just take that away,” she said. “More regulations would be fine, but what the cops are doing now feels too extreme.”
Southeast Asia is famous for its street food, delighting tourists and locals alike with tasty, inexpensive dishes like spicy som tam (green papaya salad) in Bangkok or sizzling banh xeo crepes in Ho Chi Minh City. But major cities in three countries are strengthening campaigns to clear the sidewalks, driving thousands of food vendors into the shadows and threatening a culinary tradition.
Officials say the campaigns in Thailand, Vietnam and Indonesia are largely aimed at promoting public order and food safety.
In Bangkok, the military junta has been clearing vendors from spots where pedestrians have complained about littering, sidewalk congestion and vermin, officials said, and plans to move some into designated areas that would be more hygienic.
“Bangkok wasn’t so crowded and congested” when the 1992 law regulating street vendors came into effect, said Vallop Suwandee, the chairman of advisers to Bangkok’s governor. “But now it is, so we have to reorganize and reorder public spaces.”
According to government data, Bangkok now has fewer than 11,000 licensed vendors, about half the number it had two years ago.
In Hanoi and Ho Chi Minh City, formerly Saigon, officials have led “sidewalk reclamation” campaigns in recent months that have received breathless coverage in the state-controlled news media and fueled a nationwide debate about how to regulate street vending.
And in Jakarta, the Indonesian capital, the authorities frequently evict hawkers or keep them in limbo by forcing them to pay thousands of dollars in annual “security” and “cleaning” fees that still do not guarantee a right to work. Since 2015, 17,000 sidewalk vendors have been moved into designated lots, city officials say, while an additional 60,000 or so still ply their trade wherever they can.
But in trying to modernize, these cities risk diluting their local flavor.
Eating street food was a way of life in Southeast Asia long before the region became popular with globe-trotting foodies like Anthony Bourdain and famous chefs started peddling classic street-food dishes in fancy Western restaurants.
Even today, as millions of Southeast Asian consumers develop a taste for pizza, burgers and air-conditioned shopping malls, the region’s humble sidewalk stalls still appeal to eaters of nearly all social classes. It is not uncommon for groups of businesspeople to stop their luxury cars and plop down for a curbside plate of hoi tod, fried mussels, in Thailand or hu tieu, a noodle dish, in Vietnam that costs less than a Whopper would.
“Some vendors have been selling food around here for over 10, 20 years, and I feel as though they have become cooks for my family,” Piya Joemjuttitham, a financial executive, said as he bought a mango smoothie from a sidewalk stand in downtown Bangkok.
Some street chefs have devoted followers who line up early for a dish, and legions of diners contend that the best bun cha (barbecued pork with noodles) in Vietnam, like the best khao man gai (steamed chicken on rice) in Thailand, is found on city streets.
Returning to Vietnam after years abroad was “pure happiness,” the Hanoi-based, Vietnamese-American author Nguyen Qui Duc wrote. “But few things can compare to eating bun cha. In Hanoi. On the streets.”
The fear here is that these chaotic cities will end up with sanitized food scenes like that of Singapore, a financial hub that began moving its street vendors into regulated food courts and so-called hawker centers in the 1960s, promising them financial incentives in exchange for compliance with health and safety regulations.
Peter Sousa Hoejskov, a food safety expert with the World Health Organization in Manila, said the Singapore model was among the region’s best for addressing links between street vending and food-borne disease.
But the price of the shift to hawker centers, some gourmands say, was atmosphere.
Many of the factors contributing to the changes in quality of hawker food — a rise in the use of imported ingredients, for example — would probably still have been factors if hawkers had stayed on the streets, said Cindy Gan, a food blogger in Singapore who grew up there in the 1970s.
“But what you lose is a certain cultural dynamism, I suppose, that you might associate with your childhood,” she said.
And some experts say street food is not inherently less sanitary than restaurant food. “If you’re eating fried foods or things that are really steaming hot, then there’s probably not much difference at all,” said Martyn Kirk, an epidemiologist at the Australian National University.
The W.H.O. and the Food and Agriculture Organization of the United Nations are developing a code for street vending in Asia that would establish best hygiene practices and offer broad guidelines on how governments could regulate the industry.
Several experts, however, said the recent sidewalk-clearance campaigns were a far cry from Singapore’s earlier effort because they seemed shortsighted, haphazard and biased against the poor.
“These plans are always announced by people who don’t have to worry about getting their own lunch,” said John Walsh, a professor of business management at Shinawatra University in Bangkok.
“This makes long-term suppression of street vending unsustainable,” he added, “until we reach a position, such as in Singapore and Hong Kong, where people earn enough that buying from a restaurant on a daily basis is a feasible alternative.”
Many street vendors are likely to find workarounds, dodging the police when they show up and returning to their stations later. But the cat-and-mouse routine adds an extra degree of uncertainty to an already stressful and low-paying job.
Ms. Hong, the Hanoi vendor, said her earnings had cratered by about 60 percent since the start of the crackdown, when she moved to her present location from a busy street corner as a hedge against police raids.
A vendor in central Bangkok who sells green papaya salad and would provide only her first name, Si, said she would consider returning to her hometown in Thailand’s poor northeast if the crackdown intensified.
Others have no such options.
“At my age, I wouldn’t know what else to do if I couldn’t do this,” said Nguyen Thi Nga, 55, who was selling oranges and apricots on a Hanoi sidewalk. “Even if I applied for a job, I wouldn’t get it.”