One of the attackers was held in the incident in Shache county, according to the official website of the government of Xinjiang, where mainly Muslim Uighurs are the largest ethnic group.
The website made no mention of any police casualties and said officials were conducting an investigation.
The area is known as Yarkand in the Uighur language.
Authorities say that "terrorists" have been responsible for similar attacks this year in the sprawling and resource-rich region.