Protesters marched again on Friday, and police used use tear gas and public appeals to break up crowds of Han who tried to break through to government offices.
There were no deaths in Friday's protests, Zhang said.
The protesters say the authorities have failed to stop a number of syringe attacks or to punish the Muslim Uighurs.
But Uighur leaders say members of their community have also been hurt in the attacks.
More than 500 syringe stabbings have been reported which, like the earlier violence, have targeted mostly Han.
Xinhua, the state-run news agency, quoted Zhang as saying the syringe attackers were Uighurs.
In comments broadcast on national television, Meng Jianzhu, Beijing's public security minister, said: "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. Their purpose is to damage ethnic unity."
It was the first time the authorities had suggested Uighurs were involved.
The government has, meanwhile, banned "unlicensed marches, demonstrations and mass protests" and will disperse or detain those who disobey, reported Xinhua.
Thousands of soldiers and paramilitary police were on the city's streets on Friday, leaving many areas deserted and businesses closed.
Local TV reports said that 476 people had sought treatment for stabbings, although only 89 had obvious signs of being pricked and no deaths, infections or poisonings had occurred.
On Friday, pararamilitary police with shields, sticks and submachine guns slung over their backs sealed off People's Square in the centre of Urumqi.
Groups of armed security personnel also guarded intersections in the centre of the city, blocking access to anyone without identification showing they lived or worked there.
The latest unrest has been seen as a sign that despite that crackdown, sectarian tensions remain high in the city.
"People are angry at the government, they are scared. We are living under conditions that are not normal," Zhou Yijun, a government office worker, told the Associated Press.
Another man, who gave his name only as Zheng, told the news agency people were very upset about the stabbings and described relations between Han Chinese and Uighurs as "very bad".
"These people making trouble, we catch one, we kill one," he said.
Ethnic relations in Xinjiang have grown increasingly strained in recent years, with many Uighurs resentful at the migration of millions of Han Chinese into what they see as their homeland.
The Uighurs say the Han have unfairly benefited from the mineral wealth of Xinjiang, a strategically vital Central Asian region with significant oil and gas deposits.
Meanwhile, the Han often stereotype Uighurs as lazy, more concerned with religion than business, and unfairly favored by set-aside quotas for government jobs and university places.