Chinese police on Saturday brought back 37 suspected Chinese gangsters from Angola, where they were arrested for alleged crimes against other Chinese such as kidnapping, armed robbery, extortion, human trafficking and forced prostitution.

Such crimes are a major concern for China, which has become Africa's main trading partner and a major source of infrastructure investment. As a result, many Chinese work in Africa, and their presence has brought problems such as crime among Chinese, especially in politically volatile countries that provide fertile ground for criminals.

It is a sign that Chinese businesses continue to be troubled by the country's stubborn gang culture as they expand overseas.

The Ministry of Public Security said the Chinese government sent a special police force to Angola in July which worked with local police to break up 12 Chinese gangs, resolve 48 criminal cases and rescue 14 Chinese victims, most of whom had been forced into prostitution. At home, domestic police arrested 24 other people suspected of being involved in the cases, the ministry said.

Following a request from Chinese authorities, the 37 men and women were extradited on Saturday to China, where they face trial on charges including kidnapping, armed robbery and extortion. They arrived in Beijing on a chartered plane, the ministry said.

Such crimes hurt Chinese businesses overseas and tarnish the country's image.

As a rising world power, China also wants to be seen as being capable of protecting its citizens abroad and punishing those who commit violent crimes overseas.

China Police, an online news site managed by the ministry, ran three articles on Saturday on crime fighting in Angola. In one article, Detective Liu Feng said many poorly educated, low-income Chinese became gangsters in the southwestern African country.

State media reports said they robbed Chinese businesses and kidnapped Chinese businessmen in broad daylight for ransom.

To protect themselves, Chinese businessmen in Angola hired bodyguards, purchased bullet-proof vehicles, built homes that were difficult to access and disguised themselves when they went out, the article said.

The news site said there were 14 kidnapping cases in 2011 and five people died. Out of fear, many Chinese businesses closed down in the country's capital of Luanda and elsewhere, it said.

In addition to government projects, private Chinese businesspeople go to Angola — which is recovering from a civil war that ended in 2002 — for opportunities in trade, construction, retail and food service.

Another China Police article described the experience of two Chinese women lured to Angola with the promise of well-paying jobs in a Chinese restaurant there.

Once in Angola, they were forced into prostitution, the article said.

State-run China Central Television said about 260,000 Chinese are living in Angola.