Riot police are patrolling in Urumqi in China's Xinjiang region on the first anniversary of ethnic riots that claimed the lives of almost 200 people.
The streets are reported to be quieter than usual. Thousands of CCTV cameras have been set up in public places.
A government-run website said that all police leave had been cancelled.
The clashes that erupted on 5 July 2009 between Uighur Muslims and Han Chinese were the region's worst ethnic violence in decades.
The fighting ended after huge numbers of troops were deployed in the remote western region.
After the riots, the government suspended the region's communication links to the rest of the world, including international phone calls, text messaging and the internet.
Witnesses have described a heavy police presence on the streets of Urumqi, with police vans making regular patrols of the city.
Xinjiang's public security bureau said in a statement that Urumqi was "normal", following a clampdown on illegal weapons and explosives.
A Han Chinese hotel owner told AP news agency there was a "noticeable increase in the number of police on the streets" and that his family would remain indoors as a precaution.
The man, who gave his surname as Zhang, said shoppers at the city's main market had had to pass through airport-style security checks.
Dilxat Raxit, of the overseas World Uighur Congress, said people in Xinjiang had been warned by the authorities not to hold any memorial ceremonies.
"Many people had planned to mark the occasion, which is traditional to do a year after people have died, but word came through the various neighbourhood committees that it wasn't allowed," he told AP.
A Uighur man, who spoke to BBC Chinese on condition of anonymity, described being caught up in last year's riots.
"The demonstrations were very peaceful to start with, we didn't loot anything, we didn't fight with anybody.
"But the armed police were pointing their guns at us, and shooting us ethnic minority people - that's why we fought back," he said.
The man has since left Xinjiang.
A Han Chinese man, who gave his name as Mr K, arrived in Urumqi soon after the riots when the Han Chinese staged a demonstration.
He told the BBC tensions were high and that Han and Uighurs could not even make eye contact.
"The Uighurs were very resentful of the Hans, they think it's unfair that Han people can hold demonstrations, but Uighurs cannot," he said.
China has blamed last year's violence on the local ethnic Uighur population, saying most of the recorded dead were Han Chinese.
But Amnesty International has challenged the official Chinese version of events, saying police used unnecessary force against Uighurs, followed by mass arrests and torture.
More than eight million Uighurs live in Xinjiang, which borders Central Asia.
Many are unhappy about the large influx of Han Chinese settlers which they say has increasingly marginalised their interests and culture.