Local officials blamed Rebiya Kadeer, a Uighur businesswoman who was jailed for years in China before being released into exile in the US where she now heads the World Uighur Congress, for "masterminding" the unrest.
"Rebiya had phone conversations with people in China on July 5 in order to incite, and websites such as Uighurbiz.cn and Diyarim.com were used to orchestrate the incitement and spread of propaganda," said Nur Bekri, the governor of Xinjiang.
Wang Lequan, the region's senior Communist Party official, said that Sunday's violence was "a profound lesson learned in blood".
"We must tear away Rebiya's mask and let the world see her true nature," he said.
The protest was originally called after two Uighur workers at a toy factory in southern China were killed in a clash with Han Chinese staff late last month.
"This began as a peaceful protest by young Uighurs," Seytoff said.
He said that the clashes broke out when armed police and armoured vehicles moved in to forcefully break up the demonstration, opening fire on protesters.
The clashes were the deadliest outbreak of ethnic unrest to take place in Xinjiang for several years.
About 800 people are thought to have been arrested in the wake of Sunday's clashes, with police reportedly raiding university dormitories in the hunt for others who they believe organised the protest.
The Xinhua news agency said that the situation in the city was "under control" on Monday, with a nighttime curfew imposed and paramilitary police out in force.
Local residents also reported that internet and mobile phone connections in Urumqi were unavilable - a shutdown that is becoming standard practice in areas of China hit by unrest.
"At present, the situation is still seriously complicated, Xinjiang will prevent the situation from spreading to other areas using the most powerful measures and methods and will safeguard regional stability," Nur Bekri said.
|About 800 people were reportedly arrested after the unrest [Reuters/CCTV]
One local resident contacted by the Reuters news agency said Urumqi, situated 3,200km west of Beijing, was "basically under martial law".
Witnesses said the protests had spread to Kashgar, a second city in Xinjiang, on Monday afternoon.
A Uighur man told The Associated Press news agency that he was among more than 300 protesters who demonstrated outside the Id Kah Mosque before being surrounded by police, who asked them to calm down.
China has blamed ethnic separatists and Muslim extremists for stoking unrest in Xinjiang over the past decade.
But critics of Beijing say many Uighurs are angry at political, cultural and religious persecution as well as the apparent growing presence in the region of Han Chinese - China's main ethnic group.
Local Han Chinese told news agencies that they were the victims of much of the violence in Urumqi on Sunday