(CNN) -- Ethnic Uyghur residents in Urumqi, capital of China's far west Xinjiang region, took to the streets Sunday afternoon in a rare public protest that prompted a police lockdown of the city.
China's official Xinhua News Agency said protesters attacked passersby, burned public buses and blocked traffic. The report did not say how many people took part in the protest or what their grievances were.
But a witness in Urumqi told CNN that, soon after the protest started around 5 p.m., "hundreds [of protesters] grew into easily over a thousand -- men, women and children, all ethnic Uyghurs, screaming and chanting."
Local police arrived quickly and tried to control the swelling crowd by erecting barriers in the street, but "people pushed them over," the witness said. "They were throwing rocks at passing cars and buses." As the violence escalated, hundreds of anti-riot police arrived on the scene, the witness said.
"They used tear gas and fire hoses to disperse the crowd. I saw fire trucks, ambulances, armed personnel carriers, and what looked like tanks. I heard random gunshots."
Late Sunday, the witness said Urumqi was in a lockdown, with hundreds of People's Liberation Army soldiers in the streets. He reported seeing riot police chasing protesters into alleyways and rounding up "many" of them.
The witness speculated that the protest, which took place in the predominantly Uyghur-populated Bazaar district, may have been a reaction to racial violence in southern Guangdong province.
The racial violence reportedly happened at a toy factory in Guangdong Province, where many migrants, including Uyghurs, have moved in search of work. A massive brawl reportedly broke out between workers of Uyghur and Han nationalities. Two Uyghurs reportedly died in the violent clash.
Xinjiang is home to many Uyghurs. China's constitution guarantees ethnic minorities equal rights and limited autonomy. However, ethnic tensions run deep. Minority groups like the Uyghurs complain that they are treated as second-class citizens and are subjected to discrimination by the majority Han nationalities.
"What was clear was the Uyghur protesters were not happy," the witness in Urumqi said. "They broke windows of public buses, threw bottles and rocks at the police and harassed what looked like Chinese of Han or Hui nationalities. I saw a Uyghur man kick a Han woman in the behind as she tried to get away from the crowds."
It was not clear, from official reports or the witness' account, if there were any casualties.
A spokesman for the World Uyghur Congress, a dissident Uyghur group based in Munich, Germany, told CNN that local Uyghur people in Urumqi and Xinjiang had told him by telephone that they had seen bodies thrown into military vehicles.
Dilxat Raxit added that tens of thousands of demonstrators had gathered in every Uyghur neighborhood in Urumqi to protest peacefully against what he described as the government's ethnic cleansing in Guangdong Shaoguan.
After about 40 minutes during which the crowd shouted slogans, calling the incident in Guangdong Shaoguan a planned ethnic cleansing, the Chinese military began to crack down by sending more than 50 military vehicles -- including tanks -- carrying troops into Urumqi.
All Uyghurs were ordered off the street, he said.
Sources in Kashgar said a "massive number" of Chinese PLA forces entered that city as well, and that students were ordered to remain inside.
People were also arrested along roads leading to Urumqi, he said.
"According to the Chinese law, people have the right to protest peacefully," the World Uyghur Congress said in an appeal. "We call for attention to this kind of ethnic discrimination."
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