China has detained seven company executives after suspected industrial waste discharges polluted a river with toxic cadmium, threatening drinking supplies for millions, state media said Tuesday.

The discharges have contaminated a 100-kilometre (60-mile) stretch of the Longjiang River in the southern region of Guangxi, sparking panic buying of bottled water in nearby cities, the official China Daily reported.

Jinhe Mining Co. has been blamed for dumping cadmium -- a carcinogen which can seriously damage the kidneys, bones and respiratory system -- into the river, in a spill that was discovered on January 15.

But the government has decided to widen the crackdown to go after other polluters, inspecting more than a dozen factories on the river and stopping production at seven plants.

Feng Zhennian, a local environment official, told reporters that seven executives from companies deemed responsible for polluting the river had been taken into custody, the state-run Xinhua news agency said late Monday.

He said they were all chemical plants executives who worked in Guangxi but refused to name them, the report said.

The initial spill happened in Hechi city but was now flowing downstream, endangering drinking water for 1.5 million people in Liujiang city. It was also approaching Liuzhou city, with a population of 3.7 million, reports said.

"It is a critical time right now, as downstream drinking water safety is in jeopardy," Hechi mayor He Xinxing was quoted by Xinhua as saying.

Guangxi environmental officials could not be reached for comment Tuesday.

Authorities have mobilised thousands of soldiers to dump chemical neutralisers into the river to dilute the cadmium, but levels of the chemical were still over 25 times higher than the official limit Monday in some parts.

A supermarket worker in Liuzhou, who declined to be named, said his store had been selling 2,000 bottles of water a day as frightened residents stocked up despite government pledges that the city's tap water was safe for now.

"Sales have been like this for a week. In wintertime, normal daily sales of bottled water are 100 to 200 bottles," he told AFP.

In its latest update at midday on Tuesday, the Liuzhou city government said cadmium levels were two times higher than the government standard.

Environmentalist Ma Jun, director of Beijing-based Institute of Public and Environmental Affairs, said pollution involving cadmium -- widely used in batteries -- is worrisome since it tends to persist in the water supply.

"This water pollution can be very severe, since it was caused by a kind of heavy metal that cannot dissolve naturally and is highly toxic, which could make the pollution last for quite a long period," he told AFP.

Many waterways in China have become heavily contaminated with toxic waste from factories and farms, pollution blamed on three decades of rapid economic growth and lax enforcement of environmental protection laws.

Activists say officials often turn a blind eye to industrial pollution or even collude with companies to promote local economic development.