The leak from a smelter into the Bei River north of Hong Kong was China's second environmental disaster in six weeks, following a chemical spill on a northeastern river that disrupted water supplies in China and flowed into Russia.
The area hit by the latest spill is one of China's most densely populated and is home to thousands of factories that form the heart of the country's booming export industries.
The city of Yingde, upstream from Guangzhou, stopped using water from the Bei late Wednesday, the official Xinhua News Agency said. It said the city of 210,000 people switched to supplies drawn from a nearby reservoir through a hastily installed one-mile-long pipe.
The move came after Xinhua said Guangzhou and the nearby city of Foshan were ordered to "start emergency plans to ensure safe drinking water supplies." It didn't say what the cities were told to do.
On Tuesday, the city of Shaoguan, where the smelter accident occurred, shut off running water for about eight hours.
The Bei flows into the Pearl River, which passes through Guangzhou and empties into the South China Sea west of Hong Kong.
A woman who answered the phone at the press office of Guangzhou's water department refused to say whether it has cut running water or how many people might be affected.
But the woman said only one of Guangzhou's seven water plants is close enough to the river to be affected by the pollution. She would give only her surname, Zheng. Phone calls to the city government and environmental bureau weren't answered.
The government said the spill pushed up levels of the heavy metal cadmium in the Bei to 10 times acceptable limits. Authorities in Yingde were dumping reservoir water into the river in an effort to dilute the toxins, according to Xinhua.
The disaster came after a chemical plant explosion in November spewed 100 tons of benzene and other toxins into the Songhua River in China's northeast, forcing the major city of Harbin to shut down running water to 3.8 million people.
On Thursday, the slick reached Khabarovsk, a city of 580,000 people in Russia's Far East.
The government there said it would continue to supply running water because chemical levels were within a safe range. But officials earlier warned against drinking tap water.
The twin disasters highlight China's chronic environmental problems and the precarious state of its scarce water supplies. The government says China's major rivers are badly polluted and millions of people live in areas without adequate clean drinking water.
The accidents are an embarrassment to the government of President Hu Jintao, which has promised to clean up environmental damage from China's 25 years of breakneck economic growth.
A Chinese Foreign Ministry spokesman defended Beijing's handling of the spill in its northeast, saying Thursday it was trying to minimize the impact on Russia.
Beijing has tried to ease strains with Moscow over the river spill by sending tons of activated carbon to Khabarovsk for use in water filtration plants. Chinese technicians built a dam meant to keep the toxins out of another river used by the city.
"We express our regret for the possible impact and difficulties the Russian side may have in dealing with this issue," said spokesman Qin Gang. "But we have been very quick to respond and to take measures to prevent or to minimize the pollution's impact in our cooperation with Russia."