Chinese authorities imposed tight control in the capital of Xinjiang on Friday, the fourth anniversary of ethnic rioting which killed around 200 people, said state media, exile groups and ordinary citizens.

Internet users posted pictures of baton-wielding security personnel in Urumqi, a city of more than two million people and the scene of clashes in 2009 between mostly Muslim Uighurs and members of China's Han majority that were Xinjiang's worst violence of recent years.

Last week two incidents in the region left at least 35 people dead and officials have vowed to crack down on what they call "terrorists".

"Wearing helmets, officers patrolled the capital holding guns and shields," said the state-run Global Times newspaper, describing the "heaviest security measures since 2009" in Urumqi.

There was "a heavy presence of armed police officers and police vehicles at all major crossroads in Urumqi", it added.

China commonly steps up patrols for the July 5 anniversary of the riots but the report said: "Last week's violence in the region has made authorities particularly watchful."

The date also comes in the run-up to the Islamic holy month of Ramadan.

Numerous police cars patrolled Urumqi's streets and helicopters flew overhead, a city resident surnamed Wang told AFP on Friday via an online chat service.

"I haven't heard or seen any conflicts occur, and all shops and businesses seem to be running as normal," he said.

"The Urumqi train station feels safe," a user of Sina Weibo, a Twitter-like microblogging service, posted Thursday under the name Zhangxh1970. "On the square there are only police. People who came to pick up passengers are stopped outside."

"Harmonious Xinjiang," he added, an apparent dig at the pacific image of the region often depicted in state propaganda.

That theme was highlighted on Friday evening's main national TV news broadcast, which carried segments on Xinjiang including images of dancing people in ethnic dress.

Xinjiang's population is 46 percent Uighur with 39 percent Han, who largely dominate the economy and are a majority in Urumqi.

Uighur organisations accuse Beijing of suppressing their language, culture and religion, while China says development of the resource-rich region has raised living standards.

"Despite what the West thinks about China, Beijing has for decades channelled money, technology and talent into developing Xinjiang, while respecting the culture, language and religion of its ethnic minorities," the official Xinhua news agency said Friday in a commentary.

Dilxat Raxit, spokesman for the Munich-based World Uyghur Congress WUC), said in an e-mail: "China is launching a coercion policy against Uighurs to forbid them from assembling and mosques from holding prayer ceremonies for Uighurs that died in the 7.5 incident in 2009."

"Uighurs don't feel safe even in their own homes," he added. "They have to face discriminative inspections and provocations everyday and new conflicts can be triggered anytime, anywhere."

The WUC released a report Friday claiming that at least 102 people have died in violence in Xinjiang since March, citing reports by media and Chinese authorities, though added its own sources in the region suggest the toll is substantially higher.

At the weekend, China staged a show of force in Urumqi, with armoured personnel carriers and at least 1,000 armed personnel blocking access to streets with large sections of the city centre shut down.

Yu Zhengsheng, a member of the ruling Communist party's top Politburo Standing Committee, said that China would "step up actions to crack down upon terrorist groups", while Meng Jianzhu, another senior official, called for "24-hour patrols".

State media have adopted the "terrorism" label as a key theme in recent days.

Friday's Xinhua commentary said "terror attacks" are not about religion or ethnicity but rather separatism.

"Those who want to separate China and pursue selfish goals will find any excuse to wield their knives on innocent people and set cars and buildings ablaze," it said.

The China Daily carried a lengthy front-page article Friday on last week's violence in Lukqun, where 35 people were reported killed, based on interviews with witnesses, victims and relatives of the dead.

"I can't describe how much I hate them (the attackers)," the paper quoted Adalathan Yiminiaz, the widow of a hotel owner who was killed. "My family is destroyed and I cannot see any future."

China's foreign ministry spokeswoman Hua Chunying said that Beijing had "a very clear definition" of the 2009 events.

"It was an organised, serious and violent crime involving smashing, burning and looting which was plotted by terrorists in and outside China and carried out by terrorists in China," she said.