It started with some boys fighting over fireworks. It wound up as a clash between hundreds of villagers from two competing ethnic groups.

Such incidents illustrate the ethnic tension that pervades much of China — and exploded this week in the western region of Xinjiang, taking 156 lives.

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The situation is worst in the west, the vast borderlands where Chinese imperial dynasties spilled into traditional homelands of Buddhist Tibetans, Muslim Uighurs, nomadic Mongols and Hui, a Muslim group. But other areas are not immune: The fight over fireworks broke out in February in Henan province in central China, a day's drive from Beijing.

In the recent western unrest, anger at the authorities' handling of a brawl between Uighur and Han factory workers in south China triggered a protest Sunday 1,800 miles away in Xinjiang, the Uighur homeland. Uighurs beat Han — members of the majority ethnicity in China — and torched their shops and cars. After security forces quelled the riots, vigilantes on both sides attacked people in the regional capital Urumqi.

"There is huge distrust between ethnic groups," said Nicholas Bequelin, Asia researcher for New York-based Human Rights Watch. Incidents such as the factory clash show that "people have negative stereotypes about each other, there's racism in a sense, and every community closes ranks against the 'waidiren' — the people from outside."

Government policies don't help. Beijing has promoted economic development in Xinjiang and Tibet, but it has also imposed Chinese language and culture and ignored minority grievances, blaming overseas exiles for inciting any unrest.

Many minority communities remain poor, which only hardens Han stereotypes that other groups are lazy and ungrateful, despite the government's economic assistance. The Han make up 91 percent of the population.

Even in the absence of such policies, old tensions bubble up.

February's melee in Henan province started when Han and Hui boys quarreled over fireworks. A 2004 traffic accident in another Henan village degenerated into an ethnic fight that left seven dead officially and, according to some foreign news reports, as many as 150.

Farther east in Shandong province, police shot and killed at least five Hui in a protest march in 2000 after a Han butcher advertised sales of "Muslim pork" — outraging Muslims whose dietary laws forbid the eating of pork.

Even among the Han, old feuds between clans and villages have picked up in recent years. Police deployed in March to separate two villages on the tropical island of Hainan after a fight between residents left one dead. The cause, state media said, was an 80-year-old land dispute.

Uighurs and Tibetans complain of being discriminated against when trying to get jobs and bank loans, unlike, they say, Han migrants. In Xinjiang, the Han population has soared, from 6 percent in 1949 to 40 percent in 2000.

Policies that phase out instruction in minority languages in favor of Chinese in upper grades leaves Tibetans and Uighurs feeling further disadvantaged, both in school and later in the job market. Beijing maintains the language policy is to bring these groups into the thriving mainstream.

The government also restricts religion, appointing imams and senior clerics, limiting the numbers of monks, tearing down unregistered madrassas and prohibiting minors and university students from taking part in religious services.

The government's "attitude is that Tibetans simply have to become Chinese and Uighurs simply have to become Chinese," said Andrew Fischer, an expert in development policies of western China at the Institute of Social Studies at the Hague in the Netherlands.

Beijing defends its approach, pointing to the economic progress and infrastructure Chinese rule has brought minority areas.

"The mainstream position for the last 50 years is that the minorities have benefited from Chinese peaceful liberation and being brought into the motherland and there's no problem at all," said Fischer.

The distrust of some minorities is thinly veiled. During last year's Beijing Olympics, police told hotels near Olympic venues not to rent rooms to Tibetans, Uighurs or Mongolians.

The Xinjiang riots have stirred anger among many Han, who have seen images of bleeding Han civilians on state-controlled media.

Many comments on online forums have called for a harsher crackdown. Some even say the rioters should all be shot — a comment echoed this week by Urumqi's Communist Party secretary, Li Zhi, who said that rioters involved in killings and violent crimes would be executed.