African expatriates in Beijing and witnesses claim the episode revealed a pervasive prejudice toward dark-skinned foreigners in China.

According to five bystanders, teams of police, dressed in black jumpsuits and reportedly wielding batons and taser guns, cordoned off a street in the popular Sanlitun nightclub district at around midnight and rounded up almost all the black men there. Many of the men were beaten.

The raid took place in front of hundreds of stunned expatriates outside the packed bars and clubs of the neighbourhood, which is popular with Beijing's burgeoning foreign community.

"I saw a guy being beaten by these kids. He wasn't doing anything. He wasn't fighting back," said one witness, a white American college graduate working in Beijing.

"I have not really ever seen anything so brutal," said another American. "There was blood on the streets. They were basically beating up any black person they could find."

Beijing police yesterday declined to confirm or deny the incident.

The detainees included 22-year-old Joslyn Whiteman, son of Grenada's ambassador. One witness said he saw police grab Mr Whiteman and beat him repeatedly, despite onlookers crying out, "Stop! He's a diplomat!" According to the South China Morning Post, Mr Whiteman spent the night in a hospital with concussion. "Obviously I'm very angry," the ambassador, the elder Mr Whiteman, told the Post. "My son was arrested and beaten for no reason ... I will be taking this up with the authorities and looking into the matter."

Chinese investment in developing countries, and especially Africa, has prompted a reverse migration and a growing number of African and other black people are migrating to China to study, work and learn the language. However, many African immigrants have entered China illegally and work as drug mules, a trade that is highly lucrative but which can bring the death penalty.

African dealers in Beijing offer a wide range of drugs, from cocaine to ecstasy, to a mostly expat market. Sanlitun was once a dealers' haven but has been substantially cleaned up by roundups. None, though, approached the scale or violence of that on Friday night, described as "excessive" and "chaotic" by witnesses. "I wasn't surprised [by the use of violence]," one person said. "I've seen this kind of thing happen before, but never against foreigners in front of other foreigners."

This latest raid comes as part of the effort to clean up Beijing ahead of next month's annual meeting of Communist party leaders and the 2008 Olympic Games, based in the city.

Africans say that living in China in the face of pervasive prejudice can be challenging. When Beijing rolled out the red carpet for more than 40 African heads of state last November, billboards depicting Africans clad in leopard skin underwear, and an indigenous man from Papua New Guinea, plastered the city.