Two Han Chinese residents hold a Chinese national flag as paramilitary police in riot gear block a road at the centre of Urumqi. (Reuters)

Hundreds of ethnic Han Chinese gathered in the western city of Urumqi on Thursday to protest deteriorating public safety, witnesses said.

The downtown demonstration broke the calm that officials said had returned to the city in Xinjiang region, where almost 200 people were killed in ethnic violence in July.

The new protest follows reported syringe attacks in the city and complaints that authorities aren't doing enough to protect residents.

One witness, speaking to the Cable News Network, alleged that ethnic Uyghurs have been stabbing members of the Han community with hypodermic needles since early August.

Police have arrested 15 people suspected of being involved in the attacks, the official Xinhua News Agency said.

Han resident Zhao Jianzhuang said he joined a large crowd of protesters at a downtown intersection who were being blocked by riot police from marching on central People's Square, less than two kilometres away.

Zhao said the protest was triggered by a perceived delay in trials for those arrested over the riot, as well as the series of reported stabbings. He claimed more than 460 people had been injured.

With files from The Associated Press