Major pollution of a river has forced the suspension of water supplies to the northern Chinese city of Harbin, home to 3.4m people, authorities have said.
Thousands of tons of water are being shipped in by road
"Benzene levels were 108 times above national safety levels," said China's Environment Protection Administration.
The contamination after an accident at a chemical plant is expected to pass through Harbin on the Songhua river for the next two days, officials said.
Some schools and businesses have closed and flights out of Harbin are sold out.
"Everyone wants to leave Harbin and it is very difficult to buy tickets," a factory manager told Reuters.
Benzene is a highly poisonous toxin that is also carcinogenic.
Fifteen hospitals have been placed on stand-by to cope with possible poisoning victims.
13 November Explosion at petrochemical plant, Jilin city
21 Nov Water to Harbin city cut off; local government cites mains maintenance
22 Nov State media say water could have been contaminated after the blast
23 Nov Authorities admit very high levels of benzene have been found in the water
Officials are also on alert in Russian towns further down the river.
More than 16,000 tons of drinking water is being brought into Harbin by road, the Chinese state news agency Xinhua said - though this is less than Harbin's residents normally use in a day.
The government initially said the stoppage would last four days, but a water company official has told the BBC there is no set timetable for the resumption of supplies.
BBC Beijing correspondent Louisa Lim says residents of Harbin distrust government statements, having originally been told the stoppage was for routine maintenance.
The initial announcement of water stoppages led to panic buying of water and food, exhausting supermarket supplies and sending prices soaring.
"The city was full of ridiculously large queues. People were buying water in massive quantities," English teacher Craig Hutchinson told the BBC News website.
Other residents told the BBC they felt more inconvenienced than worried.
"I can say that we feel safe and fine. Even though people... may not be able to shower, at least they can drink and cook with good [bottled] water," hostel manager Yang Yan said.
Colourless, highly flammable liquid distilled from petroleum
Used as a cleaning agent, solvent, in dyes and paints
Lethal to humans exposed to it in high levels
Chronic exposure leads to progressive degeneration of bone marrow and leukaemia
The order to cut off the water comes after a 13 November explosion at a petrochemical plant in Jilin city, about 380km (230 miles) up the Songhua river from Harbin.
Five people were reported to have been killed in the blast, and more than 60 injured.
The explosion forced the temporary evacuation of some 10,000 residents, but people have since been allowed to return home.
The Beijing News showed pictures of dead fish washed up on the banks of the Songhua river near Jilin city, but the authorities said there was no sign that chemicals in the river had contaminated the water supply.
Local media showed pictures of dead fish in the Songhua River
The high levels of benzene were found upstream of Harbin, but the contaminated river water was expected to reach the city on Thursday morning and take two days to pass through.
Officials insisted enough water would be brought in to meet the needs of residents.
Environmental officials in Russia said they were also monitoring the Amur river, which is fed by the Songhua and is the main water source for the city of Khabarovsk.
Harbin is in China's north-east Heilongjiang province, and is one of the country's coldest cities, with overnight temperatures this week falling to -12C.
Are you in the area? Have you or anyone you know been affected by the decision to cut off the water supplies? Send us your comments using the form below:
My son is an exchange student in Harbin. They have been told not to use the water for 10 days including showers, etc. Water from local wells are OK. All water is sold out, but fortunately, the locals haven't discovered Gatorade. Any confirmed details would be appreciated.
Charlie Goodrich, Lexington, MA, USA
At first glance, the Harbin city authorities look not candid enough towards the residents. However, compared with all the other cities along the Songhua river between Jilin and Harbin, all of which should have already been affected by the polluted river water and yet have been quiet on this, the Harbin city authorities are still doing a better job.
theo (Chinese national currently living in Singapore),
I am just waiting for any news on this. My sister is in Harbin teaching and sent me a flurry of disturbing emails- the most perturbing is that there is brown oily water that smells of urine coming out of her pipes. Also some officials have left the city where she is. She has yet to respond to my seven emails and I am eagerly awaiting any news. She also said that she believes that they have known about this water situation days before announcing it.
Kem Kramer , Toronto
I have my oldest brother who is currently working in Harbin, China. We contact him every couple of days and he has let us know of the devastation that is occurring due to there been no water. He has been told the shower he had the day before yesterday is the last one until the end of the week at the earliest. He lives with four other English boys, as they are all teachers working to teach Chinese children English. They had been told that there was no water but not given enough time to save clean water for them to live from. There seems to be a lot of anguish across the country but at least we know he is well so far. we just hope that the showers they all had when the chemicals went into the system will not harm them in any way.
Victoria Hutchinson, Redcar, England
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.