Guangdong police cracked 83 cases involving transactions through secret private banks between April and December, seven times more than all of 2014.

Police uncovered 79 illegal sites and detained 231 suspects.

"The private banks investigated have become a channel for online gambling, telephone fraud, moving suspected drug traffickers' money in and out of the mainland and domestic corrupt officials sending their bribes abroad," according to Huang Shouying, director of the economic crime investigation bureau under the Guangdong provincial Department of Public Security.

Police seized 37.1 million yuan ($5.72 million) in cash and froze 3,491 accounts with total balances of 665 million yuan.

Guangdong, one of the country's economic powerhouses, has been on the front line in the fight against illegal private banks.

"Active underground private banks have damaged normal financial operations in the province," Huang said.

Twenty of the cases involved at least 100 million yuan each.

Police in the Shenzhen special economic zone cracked a case involving 44.2 billion yuan in August and another case involving 12 billion yuan in June.

Police in Foshan busted a case valued at more than 20 billion yuan in June and police in Zhuhai had a case involving more than 3.8 billion yuan.

Most of the cases involving illegal private banks were in Guangzhou, Shenzhen, Zhuhai, Dongguan, Foshan and other prosperous cities in the Pearl River Delta, Huang said.

In Shenzhen alone, police busted 32 cases from April to December.

In Foshan in June, two suspected corrupt officials from Jiangxi province were detained after they illegally sent more than 12.8 million yuan in bribes to a bank account in Macao via a local private bank.

Police in Guangzhou, Shenzhen and Dongguan also helped detain three suspected corrupt officials when they cracked down on the private banks.

The suspected corrupt officials used the private banks to send their bribes abroad, said Huang, who did not provide details of the cases.

The private banks usually charge their clients service fees, Huang added.

Huang said the crackdown has dealt a heavy blow to the activities of the province's secret private banks and has helped ensure good financial order in Guangdong.

But he admitted that fighting secret private banks is a long-term and tough task in Guangdong.

Huang attributed the growing number of illegal private banks in Guangdong to the province's rapid economic growth, active financial activities and proximity to Hong Kong and Macao.

Huang promised that more special operations in cooperation with People's Bank of China, the High People's Court and the High People's Procuratorate will be launched to fight private banks.