Taps were turned off in Harbin, capital of northeastern Heilongjiang province, at midnight on Tuesday after there had already been panic-buying of bottled water and food.
"Everyone wants to leave Harbin and it is very difficult to buy tickets," an unnamed factory manager told Reuters.
"All containers are being used to store water, including the bathtub. It will be okay for four days, but not longer than that."
The water supply was shut down after a blast at a chemical plant on 13 November in neighbouring Jilin province, only a few hundred metres from the Songhua River
The river supplies water to Harbin, a metropolitan area of nine million people.
The Beijing Times newspaper said the pollutants in the partly frozen river included benzene, an industrial solvent and component of petrol, and that they had reached the city.
"Pollution is definite," said a regional water official, who declined to give his name.
"It has entered the Songhua River and has affected the banks and lower reaches."
Pollutants in the river includes
Benzene, an industrial solvent
But state television said no pollutants had reached Harbin by 11am (0300 GMT).
Fifteen hospitals were on standby to take in contamination victims, the state-run Xinhua news agency said.
A government notice saying supplies would resume in four days have been removed, raising doubts about how long the crisis would last.
A Harbin government spokesman told Reuters: "The new notice does not necessarily mean an extension."
"But we will make a decision after four days according to the water quality at that time," the spokesman said.
"There is sufficient water. Residents have all stored a lot and we have been rushing in water from other places. We also have safe underground water."
"There is sufficient water. Residents have all stored a lot and we have been rushing in water from other places. We also have safe underground water"
Prices of bottled water have soared in recent days and state media said shops had been ordered to restore prices to their level before panic-buying set in.
US Budweiser beer maker Anheuser-Busch Inc, which owns a stake in Tsingtao Brewery Co and runs a brewery in Harbin, said it had not been affected because it uses well water.
"We can reassure consumers that our beers our fine and safe," it said.
The Songhua River runs into Russia after several hundred kilometres.
A Foreign Ministry spokesman said on Tuesday he did not know the extent of the pollution, if any, but that China always took care of other countries' border water interests.
More than 16,000 tonnes of bottled water was being transported to Harbin from nearby cities and provinces, Xinhua said.