BEIJING - Chinese police have arrested 32 suspects who allegedly produced cooking oil recycled from the waste and sold them to restaurants in a number of provinces in the country's first major crackdown on notorious cooking oil scams.

More than 100 tonnes of the illegal cooking oil, made from leftovers dredged from gutters behind restaurants, were seized after police bust a criminal network spanning 14 provinces, the Ministry of Public Security reported Tuesday.

"In less than two years, more than 10,000 tonnes of the 'gutter oil' had entered the market. We are still probing the case," said Hong Jufeng, a police officer involved in the crackdown.

Police began their investigation in March after residents in Ninghai County of eastern Zhejiang Province reported that a group of people were buying leftover oil from local restaurants.

The 32 suspects were caught in a cross-province raid in mid-July, when police from Zhejiang, Shandong and Henan jointly busted six underground workshops and two illegal production lines, police authorities said.

Leftovers were first collected and processed in open-air giant pots to extract "crude gutter oil", Hong said while explaining the operation of the network.

In the village of Shuiyang in Ninghai, a giant pot which once was used to process the leftovers is left in woods.

"It really stinks," said Yang Genmu, a villager of Shuiyang. "There were rotten meat, seafood and bones in leftovers."

The oil would then be refined by a Shandong-based oil recycling firm licensed to produce bio-diesel only. The refined "cooking oil" was sold to regional wholesale companies and further distributed to small eateries and food-stands in rural areas, police said.

"The network covered a wide region and worked under cover, posing tremendous challenge to police investigation," said Wang Weibiao, deputy police chief with the city of Ningbo.

Wang said while recycling oil from the waste is not banned but selling the recycled oil as cooking oil is an act to be punished by law.

Managers of the Ge Lin Co., Ltd., the Shangdong-based oil refinery firm, told the police that they illegally produced cooking oil as its market price was much higher than diesel.

Police investigators said the company was run with high-level of secrecy that it is difficult for outsiders to learn what the company was really producing.

Ge Lin's managers accused people with the wholesale companies of accomplice as the refined "gutter oil" look and smell different from the cooking oil and it was sold at lower prices, police said.

Samples of Ge Lin's refined "gutter oil" was tested to be carcinogenic by Beijing's food safety watchdog in August, police added.

The "gutter oil" scam was the latest to fall under Chinese authorities' serious efforts to ensure food safety in recent months.

Police said the bust of the criminal network was important to push forward the national crackdown on similar cases. They also urged food watchdog authorities to tighten supervision over the production of cooking oil.