* Protests over handling of alleged syringe attacks
* Rare public demands for Communist Party chief to resign
* Unrest shows continued ethnic volatility in Xinjiang
(Changes dateline, adds details of protests, background)
By Lucy Hornby
URUMQI, China, Sept 3 (Reuters) - Protesters massed in the capital of China's far western region of Xinjiang on Thursday demanding the ousting of its top official, amid a scare over syringe stabbings that has reignited ethnic tensions.
The demonstration was a rare public challenge by Han Chinese to the ruling Communist Party in Xinjiang, a tense region where deadly ethnic strife with Muslim Uighurs broke out in early July. [ID:nPEK148344]
"Resign Wang Lequan, the government is useless!" protesters shouted outside the regional government building in Urumqi, referring to Xinjiang's Communist Party chief, who has held office for 14 years. Others called for his execution.
"Wang Lequan apologise to the Xinjiang people," they yelled, some of them lobbing bottles and other objects in Wang's direction, a witness, who requested anonymity, told Reuters.
He estimated that by early afternoon 3,000 people gathered at the People's Square in central Urumqi. China's Xinhua news agency said that tens of thousands were involved in protests across Urumqi.
A photograph showed Wang using a microphone to address the crowd from a balcony. The witness said Wang told the crowd that 30 people had been detained over the claimed syringe stabbings.
Xinhua said the protests began in the morning after claims that a man had stabbed a five-year-old girl. The reports did not say whether the accused attacker was Uighur.
Some witnesses said they saw Uighurs, a Turkic-speaking Muslim people native to the region, being beaten by Han Chinese.
A resident said he had seen a group of Han Chinese protesters beating a Uighur suspected of carrying out attacks with syringes, but he was rescued by police and taken to hospital.
The Xinjiang health office said that over the past two weeks 476 people have gone to hospital to report apparent syringe stabbings -- 433 of them Han Chinese -- and doctors have "found clear syringe marks in 89 cases," regional television reported.
"Most of the injured members of the public realised what had happened only after the event and then reported it, making it difficult to solve the cases," said Zhu Hailun, head of the Communist Party's law-and-order committee in Xinjiang, according to China News Service.
Nicholas Bequelin of New York-based Human Rights Watch said it was still unclear how much truth lay behind the claims.
"These kinds of rumours do happen in China after unrest," he said. "There's always bizarre rumours that spread after violence."
The People's Liberation Army has sent doctors to Xinjiang to assess the wounds of people who said they had been stabbed, the Chinese Ministry of Defence said on its website.
Police have seized 21 suspects and formally arrested four of them, the Xinhua report said, without saying what they were charged with.
In Xinjiang's worst ethnic violence in decades, Uighurs attacked majority Han Chinese in Urumqi on July 5, after taking to the streets to protest against attacks on Uighur workers at a factory in south China in June in which two Uighurs were killed.
Han Chinese in Urumqi sought revenge two days later.
Human Rights Watch's Bequelin said the explosion of anger over the claimed syringe attacks showed public anger over the July killings had not evaporated. At least 197 people died in the violence, authorities say.
"It does show that the government hasn't adequately explained the failings of the security forces on July 5," said Bequelin. "This illustrates the deep distrust between the (ethnic) groups."
Dilxat Raxit, a spokesman for the exiled World Uyghur Congress said local sources had told him 10 Uighurs had been injured in the new protest, and some shops were attacked.
But in this latest unrest, Han Chinese also saw themselves as victims.
"Han Chinese are complaining about the worsening social order," said one hotel worker in the city. "They resent the Uighurs for the stabbing thing."
Rumours of AIDS patients attacking people with hypodermic needles have previously rattled parts of China, but were later shown to be unfounded.
By late at night, Urumqi was outwardly calm with security forces blocking roads in parts of the city and police and security forces on patrol.
The unrest welled up less than two weeks after Chinese President Hu Jintao visited Xinjiang, urging stability and economic growth there. [ID:nPEK81652]
It also erupted at a sensitive time for China, preparing for the 60th anniversary of the founding of the People's Republic on Oct. 1 with a nationwide security clampdown.