Xinjiang police have sent a text message to Urumqi residents warning them to watch out for syringe attacks.
The message was sent out several days ago in response to incidents that have emerged in the past week involving harmful injections, said two police officers who answered telephone inquiries.
The city's residents are still recovering from the mainland's worst ethnic violence in recent years in July, and the message struck home, with many residents staying off the streets.
Businessmen from other parts of the country scrambled to leave the city, where they were due to take part in the Urumqi Fair, being staged from Tuesday to Saturday.
Police refused to say with what the needles were contaminated and whether there had been any casualties. But rumours have spread that the syringes could be infected with HIV or anthrax, and that many people had been admitted to hospital.
Several Urumqi residents who received the message, sent via the city's propaganda department, said officials had called on people to take precautions against assaults while walking in the streets.
The message said the police department had recently solved a case of assault involving injections. A policewoman said the message was sent out as a precautionary measure and there was no need to panic.
But many residents said they were now afraid to walk around the city.
One Uygur woman, who declined to be named for fear of reprisals, said tensions in the city had appeared to intensify as police returned to patrol the streets. 'A friend of mine works for the hospital. She told me some people were sent there after being injected,' she said.
Clashes in Urumqi erupted on July 5 and involved at least 1,000 Uygurs in what began as a protest, but led to confrontations with police that escalated into attacks on Han Chinese, triggered by another riot in Guangdong. Nearly 200 were killed, according to the government's account, and more than 1,700 injured.
Residents had been reassured that stability and harmony had been restored in the restive region. Kurexi Maihesuti, a vice-chairman of the autonomous region, said yesterday in Beijing that tourism and other business were picking up again with the Urumqi Fair - the biggest trade fair in Xinjiang - attracting a similar number of visitors as in previous years.
However, a Han driver, whose clients are mainly tourists, said he had taken three businessmen attending the fair to the airport yesterday. He said they had left after hearing about the needle attacks.
'The situation is worse than in July. There are so many rumours and each of them sounds scary,' he said. 'The streets are still very quiet and the tension here is no less than in July. We've received many calls from parents wanting to rent our cars to send their children to schools. But not many want to do the job.'
It has been reported that trials of about 200 people arrested in the rioting would begin soon. Maihesuti said the judicial process was proceeding according to the law.