The arrests were made after the police said to have detained 1,572 people in connection with the clashes in which 197 people were killed and hundreds were injured.
The statement from the Urumqi Public Security Bureau said late on Sunday the 319 people had been detained after information was received from the public or obtained through the police investigation.
"They were arrested in Urumqi or in other parts of Xinjiang," the statement said, adding the held suspects would face charges for instigating violence and causing death and injuries to hundreds of people.
Days after the violence, the public security departments of Xinjiang and Urumqi issued a notice urging local people to turn in suspects who were still at large.
"Every suspect, without any exception, will be arrested and punished according to the law to root out any hidden danger," it said.
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