A huge swathe of toxic water has reached the north-eastern Chinese city of Harbin after drifting down the river which is the city's main water source.
The authorities have been ferrying in water by tankers
Massive amounts of the chemical benzene were released by a blast 11 days ago at a plant in Jilin, about 380km (230 miles) further up the Songhua river.
Authorities shut off water supplies to Harbin's 3.8m residents two days ago.
There is plenty of bottled water, wells are being dug and supplies are being driven in, says a BBC correspondent.
More than 16,000 tonnes of water are also being brought in by road, the Chinese state news agency Xinhua says - though this is less than Harbin's residents normally use in a day.
At water collection points, people are queuing up with plastic buckets, basins, kettles and bottles.
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
Fifteen hospitals have been placed on stand-by to cope with possible poisoning victims. Some schools and businesses have closed and flights out of the city are sold out.
"Everyone wants to leave Harbin and it is very difficult to buy tickets," a factory manager told Reuters.
It is possible that people upstream would not have known anything was wrong with the water, our correspondent says, raising concerns that farmers and other residents may have been using it as normal.
Reports on Thursday said the contaminated body of water had levels of benzene more than 30 times higher than was considered safe.
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
China's environmental watchdog had found levels in the river 108 times the safe limit on Wednesday.
The 80km (50 mile) contaminated stretch of water reached Harbin at about 0300 local time on Thursday (1900 GMT Wednesday), the government said. It was expected to take 40 hours to pass.
"After it passes... we will have to make efforts to disinfect the water," Shi Zhongxin, director of the city's water bureau, said on state television. He did not give any details.
Neighbouring Russia was urgently seeking information from China on the spill.
A state of emergency will take effect in Russia's eastern Khabarovsk region, about 700km (435 miles) downriver from Harbin, on Friday, amid fears of possible contamination of the Amur river, officials from the emergency ministry say.
They said it would affect Khabarovsk on 30 November-1 December
However, the Chinese authorities played down the threat to Russia, saying it would take longer than that to affect the country.
"It will be about 14 days before the polluted current flows into the Heilong river [the Chinese name for the Amur]," Zhang Lijun, the State Environmental Protection Administration's deputy director, told reporters.
"It is weakening and the level of impact will further decrease," he said.