Beijing police and environmental protection officers raided 107 locations in Fengtai district this week to crack down on illegal polluters.

Evidence of illegal activity was uncovered at 40 sites, the city government said in a statement on Wednesday.

Six enterprises were immediately closed down, with five cases potentially carrying criminal punishments, the authority said. It did not state how many suspects had been detained.

Fengtai is a largely industrial area southwest of downtown Beijing, although it also has many farms. The sites were identified using a "grid scanning technology" or through tipoffs from the public, the authority said.

China Daily was invited to join inspectors on Monday, the first day of the crackdown.

After visiting a pig farm, where the owner had been illegally dumping waste, the officers set out to find a secret workshop in the mountains that had been producing recycled oil, also known as gutter oil.

"It wasn't easy to find the place, as it was hidden deep in the mountains. A villager finally helped us find it," said Feng Zhijun, an environmental protection officer. "When we got there, the gate was locked and no one answered the door. There as a nasty smell in the air, and the land was greasy."

Police broke through the perimeter and found a narrow path into the yard.

"We found eight large containers of oil. They still felt warm," Feng said, adding that the workshop had been abandoned. "The people may have run away when they heard us.

"We don't know where the oil was to be delivered to."

Zhang Yi contributed to this story.