"The needle stabbing incident is a continuation of the '7-5' incident, and it's plotted by unlawful elements and instigated by ethnic separatist forces," Meng said in comments broadcast on nationwide television. "Their purpose is to damage ethnic unity."
Meng provided no evidence to back up his charges, nor has the government substantiated accusations that separatists incited July's violence. By most accounts, the riot started after police confronted protesters from the Muslim Uighur ethnic group, who then attacked Han Chinese.
But the Xinjiang region, where Urumqi is located, has for decades faced a simmering separatist movement by Uighurs, a largely Muslim ethnic group.
Meng's comments were the first time authorities suggested Uighur militants were involved in the stabbings that had fed rumors and fear in the city for days but that were first publicly reported Wednesday.
Meng's dispatch to Urumqi was a measure of Chinese leaders' anxieties that order was slipping in the often tense city of 2.5 million and that violence between Hans and Uighurs could flare anew.
Heavily Uighur neighborhoods were sealed off by security forces forming barriers at street entrances.
The official Xinhua News Agency said Meng was tasked with "defusing the ongoing unrest in the city." China Central Television showed him visiting police, troops and local residents.
In a nod to public anger, Meng vowed the government would speed up the process of charging and prosecuting the more than 1,200 people detained in the July riot.
"We should quicken the pace of dealing with the detained suspects and dig up the plotters behind this, and severely punish the murderers," Meng said.
Local police authorities said Friday that hospitals in Urumqi are now treating 531 people who believed they were attacked by hypodermic needle, 55 more than previously reported, Xinhua said. About 106 of them showed obvious signs of needle attacks, it said. Most of the victims were Han Chinese.
So far, none of those stabbed showed any signs of infection or poisonings, state media has said. Infection by AIDS is a concern, given Xinjiang's high rate of HIV cases, spread by needle-sharing among drug users.
None of 160 or so people treated at the Urumqi Medical College Hospital showed symptoms of AIDS or hepatitis, said Lin Fangmu, director of the preventative medicine department.
The most common symptom they displayed was "mainly just fear, terror," Lin said.
Undeterred by an overnight curfew and police cordons in the center of the city following Thursday's demonstration, hundreds of young Han Chinese men protested outside the headquarters of Xinjiang party secretary Wang Lequan — an ally of President Hu Jintao_ chanting for him to step down.
Armed police in riot gear marched into the crowd to push people back. After several surges, the police broke up the demonstrators, some of whom were singing the national anthem.
The smell of tear gas drifted over parts of the city, and CCTV showed footage of tear gas drifting through big crowds.
Hong Kong Cable TV reported that paramilitary police had fired tear gas to disperse a crowd of about 1,000 near the city government offices Friday morning.
One Han man, who brought his 9-year-old daughter because her school was closed, argued with paramilitary police.
"It's been two months already. How many more months are we going to wait, how long before us Han can feel safe?" said the man, who would give only his surname, Ma.