Police on the mainland have detained eight men suspected of being involved in a major kidnapping case in Hong Kong.

Ransom of HK$2.83 million ($365,200) and stolen watches, jewelry and other valuables were seized in an operation involving officers from the mainland and the city, said Guo Shaobo, deputy director-general of Guangdong province's Department of Public Security.

The arrests came after a suspect surnamed Zheng was detained by Hong Kong police at the Luohu checkpoint on May 3 as he tried to enter Shenzhen in the province.

"Police have detained a total of nine suspects allegedly involved in the case so far," Guo told a news conference in Guangzhou, capital of Guangdong, on Thursday.

"All the detainees, including the suspected gang leader surnamed You, are mainlanders," Guo added.

You, born in 1976, and a suspect surnamed Wang were seized in Guangdong's Huidong county on May 9, Guo said. Police recovered the ransom money after detaining the pair, who had allegedly entered Guangdong on May 6 with the cash.

Officers arrested the six other suspects in Shenzhen and Dongguan, also in Guangdong province, and Weng'an in Guizhou province.

Police say a woman surnamed Law was kidnapped and more than HK$2 million in cash plus watches, jewelry and other valuables were stolen at a villa in Hong Kong on April 25.

The hostage, the granddaughter of a local tycoon, was held in a hilltop cave on Kowloon Peak and was released after her family paid the kidnappers HK$28 million.

Guo said that officers are still trying to recover the remaining HK$25 million ransom money, and that they are investigating whether any Hong Kong residents were involved in the kidnapping.

Li Ping, director of the criminal investigation detachment at the Shenzhen Bureau of Public Security, said seven of the suspects carried out the kidnapping and the two others took the cash and valuables to Shenzhen.

A special task force was set up on April 30 as Hong Kong police sought help from their counterparts in Guangdong, because they suspected that the suspects had escaped to the province, said Guo, who headed the team.

"This case demonstrates the successful cooperation between Hong Kong and Guangdong police in fighting cross-border crime," Guo added.

zhengcaixiong@chinadaily.com.cn