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.