Tens of thousands of Han Chinese demonstrated on Thursday in central city of Urumqi, the capital of China's Xinjiang province, calling for the resignation of the region's leader, Wang Lequan, who has held the post for 14 years.
Deputy Mayor Zhang Hong said Twenty-one Uighurs had been arrested on suspicion of prinking people with tainted needles, adding that four have already been indicted.
Han Chinese protesters massed for a third day in the capital of the Xinjiang region, to protest that authorities had failed to control a spate of syringe attacks and had been slow to bring to trial ethnic Uighurs charged with deadly rioting on July 5.
These Uighurs have been stabbing us with needles, said a man trying to push through barriers sealing off a Uighur neighborhood. We need to take care of the problem.
Urumqi hospitals are treating 531 victims of syringe stabbings, the Xinhua news agency said, with 106 of them showing obvious signs of needle attacks.
Rumors of AIDS patients attacking people with hypodermic needles have previously rattled parts of China, but were later shown to be unfounded.
Thousands of angry crowds confronted paramilitary troops and police at intersections, demanding more rights for Han people, the report said.
China says Uighurs campaigning for independence are allied with Islamist militants in the region.