Many of the protesters blame ethnic Uighur Muslims for the stabbings.
Riot police used tear gas to disperse angry crowds earlier, and China’s top security official has arrived in Urumqi to try to restore order.
In July about 200 people – mostly Han Chinese – were killed in ethnic riots in Urumqi.
Chinese authorities blame Uighur separatists for July’s violence, saying it was orchestrated by Uighur separatists in exile.
Xinjiang’s population is split between mainly-Muslim Uighurs and Han Chinese – the country’s majority ethnic group.
Zhang Hong, vice-mayor of Urumqi, confirmed to reporters that there had been casualties in the latest unrest.
“On Thursday, 14 people were injured and sent to hospital and five people were killed in the incidents including two innocent people,” he said.
Mr Zhang gave no further explanation of how the people were killed.
Meanwhile, Beijing has sent its top security official, Meng Jianzhu, to Urumqi.
On his arrival he was quoted by state-run news agency Xinhua as saying the syringe attacks were a continuation of the July unrest.
He said the attacks were instigated by ethnic separatist forces seeking to undermine ethnic unity and urged local officials “to restore social order as soon as possible”.
But analysts say Mr Meng’s visit reflects Beijing’s desire to take control of the situation – and suggests the central government no longer trusts local officials.
‘All marches banned’
The BBC’s Michael Bristow, in Urumqi, says hundreds of police fanned out across the city on Friday, breaking up angry crowds gathering at road junctions.
Tear gas was fired to disperse one group of protesters and there were scuffles at a crossroads as police moved in to arrest several demonstrators.
Xinhua reported that the authorities had banned “all gatherings, marches or protests on roads or other public venues”.
The crowds demanded more protection amid media reporters saying more than 500 people, almost all Han, have sought treatment for stabbings in the past few weeks.
There is also anger over what many perceive as official slowness in punishing suspects charged over July’s riots.
Tension between Xinjiang’s Uighur and Han communities has been simmering for many years, but July’s ethnic unrest was the worst in China for decades.
It began when crowds of Uighurs took the streets to protest about mistreatment – but their rally spiralled out of control and days of violent clashes followed.