SHENZHEN, China -- Chinese officials are trying to tighten their control over millions of migrant workers who have been the engine behind this nation's spectacular economic growth but who now are emerging as the biggest threat to its stability.


The country's itinerant masses -- as many as 100 million people, equivalent to the population of Mexico -- are adrift in China and have sorely strained traditional government controls. Most migrants are not registered, as required, with authorities. Many don't pay taxes. Some are now blamed for creating crime waves in China's cities.


And as the workers' numbers grow, so do tensions between those who are behind China's boom and those who are reaping the benefits.


In a belated crackdown on the "floating population,'' the government now hopes to prevent violent outbursts such as a recent riot here in China's showcase "economic zone'' that left 11 workers dead.


The clash began, as one worker said later, over "just a little thing'': A wealthy villager rode his motorcycle through a blockade onto a road freshly tarred by migrant workers. The angry laborers reacted by trashing his bike. The villager, in response, called police on his cellular phone. The workers summoned friends from nearby construction sites. In the violent battle that followed, 11 workers were killed by paramilitary troops armed with submachine guns.


"The police treat us workers no better than dogs,'' a man from inland Hunan province, who was injured in the melee, said bitterly from his hospital bed.


Since paramount leader Deng Xiaoping's 1979 economic reforms freed this nation's peasants to find their fortunes in the cities, migrant workers' cheap labor has driven China's phenomenal expansion. The economy has grown at an average rate of 10 percent a year since 1982.


Chinese farmers, in turn, have found more than freedom of movement: Not only can they nearly triple their countryside wage by moving to the cities, they can evade taxes and China's one-child policy in the process.


But as China struggles to balance its rapid economic development and the social shifts that go with it, it must also confront officials' greatest fear -- that they may lose control of the once tightly regulated population.


Along with their possessions, packed in rice sacks and fertilizer bags, the "floating population'' brings the specter of chaos and crime. Authorities in the southern province of Guangdong -- where there is one migrant for every four permanent residents -- claim that two-thirds of reported crimes are committed by migrant workers.


Though outsiders are often made scapegoats for existing ills, and many say the threat they pose is overblown, the government -- which once encouraged idle farmers to come to the cities -- now is telling them to turn around and go home.


"It is unavoidable that the movement of rural surplus labor will be blind and disorderly during a certain stage," said Vice Premier Wu Bangguo.


"However, if we let such a situation go on, it will jeopardize social stability and obstruct the progress of our reform."


Economists argue that the free-flowing labor force actually is good for social stability, because workers' remittances to China's backward, inland provinces helps ease the growing gap between the rich and poor. It also helps to gradually elevate women's status, as rural girls who go to cities prove to be money-earners, not just mouths to feed and marry off.


But such longterm benefits don't alleviate tensions now between the scrappy army of rural workers and local residents who consider the outsiders a drain on city resources -- or a menace.


Several days after the Shenzhen riot, a group of workers from Hunan province huddled outside what they call "the cave'' -- an underpass converted into a temporary barracks filled with beds on bamboo stilts. They debated whether to finish their work on the road or return home. A passing police truck sent a bristling ripple through the crowd, whose members looked around skittishly, ready to bolt, until the car drove on.


"I think we should stay,'' said a youth with a faint mustache, who noted he likes making $81 a month for roadwork -- twice what he made on the family farm in Hunan. "But I may change my mind if something else happens.''


The transient life, group members said, is made better by camaraderie -- some workers have been in Shenzhen together for two or three years -- and by the gaggle of prostitutes who faithfully follow the workers' camp and cook for them.


But the leader of the group, a 40-ish man who declined to give his name, expressed deeper cynicism, saying: "The police are always very rude to us. I don't know why I stay here and leave my family behind. I came just to open my eyes. Well, now they're open.''


Workers complain of discrimination by city dwellers and constant harassment by police, who stop them to check for identity papers and beat them without provocation. They vented their pent-up resentment during the riot here, rampaging through the local Public Security Bureau headquarters, breaking every window in the three-story building and smashing furniture. They didn't stop until military reinforcements trapped rioters in a courtyard and opened fire.


"If the military police hadn't come, the local officials would have been killed,'' said an office worker from Hunan province who witnessed the scene. The riot was unusual. But authorities, fearful of migrants' growing autonomy and assertiveness, are ratcheting up controls nationwide.


In Shanghai, companies employing out-of-town workers must contribute to a welfare fund to cover growing costs for social services for itinerant workers; they must pay heavy fines if caught using laborers without official work permits.


In Beijing, where political gestures take precedence over commercial considerations, a recent crackdown on migrant communities began with a vituperative media campaign.


"Many migrants spit everywhere, throw cigarette butts anywhere and pay no attention to sanitation," wrote the official Enlightenment Daily recently. "The smell from the migrants' quarters is inescapable. Quite a few of them are criminals engaged in gambling, prostitution, rape, murder, blackmail and looting."