Four people, including a police officer, and an unconfirmed number of rioters were killed on Monday after a police station was attacked and set on fire in Hotan, the Xinjiang Uygur autonomous region, local police said.

Eight people were taken hostage by the rioters. Two were later killed.

Police managed to free six hostages after killing several rioters about 90 minutes after the station was attacked, Yang Guoqiang, a senior official with the regional publicity department, said.

The attack took place just days after the second anniversary of the July 5 riots in Urumqi, in which 197 people were killed and 1,700 injured. The riots resulted in extensive damage to property and vehicles.

"A group of Uygur rioters attacked the police station," Yang told China Daily on the phone.

"No group or organization has claimed responsibility.

"It's obviously a long-planned, unprovoked attack."

Police and publicity officials in Hotan refused to release any more information other than what was carried in a brief report by the Xinhua News Agency.

Hotan is located at the southern part of the autonomous region. About 96.3 percent of Hotan's population are Uygur and 3.5 percent are Han.

A source told China Daily that the attack occurred at about 12 am at the grand bazaar police station, or Naarburg Street police station, in a busy commercial district.

The grand bazaar has a variety of goods on offer, including fruit and livestock, and is a popular meeting point.

The source, who declined to be named, said the two hostages killed were civilians visiting the police station.

A paramilitary police officer and a security guard were also killed. Another security guard was seriously injured, Xinhua reported.

Police from other stations brought the situation under control at 1:30 pm and freed six hostages. The attack on the station is currently under investigation, Xinhua reported.

The national counter-terrorism office dispatched a team to Xinjiang.

A number of police stations refused China Daily's interview requests, but Muhtar, a police officer at Wana village station in Lop county, Hotan, said police had been asked to step up vigilance.

"After the attack, authorities asked us to step up patrols and be ready to go on duty," Muhtar, who joined the local police force in 1997, said. "Such attacks are very rare here."

Li Wei, a Beijing-based anti-terrorism expert, said he was "shocked" by the attack.

"It exposed problems in local public security. The crackdown (on terrorists) should be further strengthened."

In 2008, two terrorists driving a truck in Kashgar threw home-made explosives at policemen undergoing morning drill, killing 17.

Ministry of Public Security spokesman Wu Heping said the two attackers belonged to a terrorist group led by the East Turkistan Islamic Movement, which was broken up by the police in 2010.

Mao Weihua and Cui Jia contributed to this story.