A blast at a Chinese plant 12 days ago sent the equivalent of 10 tanker-loads of lethal substances into the Songhua river, Chinese state media report.
Harbin has brought in more than 16,000 tonnes of water
About 100 metric tons of toxic chemicals entered the river as a result of the blast in Jilin, and the leak is now passing through the city of Harbin.
All taps are off in Harbin but the water supply may be restored on Sunday after being shut down for three days.
The company behind the blast has apologised for the accident.
Harbin's 3.8 million residents are undergoing their third day without water.
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
An 80-km (50-mile) contaminated stretch of water reached Harbin at about 0300 local time on Thursday and was expected to take 40 hours to pass.
Levels of the chemical benzene are ten times higher than considered safe.
There is plenty of bottled water, wells are being dug and supplies are being driven in.
But one local paper says some people have been trying to steal water to fuel their heating systems.
Another reports that inhabitants were still fishing, despite the threat to their health.
Officials are hoping the poisonous chemicals will dissipate as they flow down the river towards Russia.
The toxic leak is now expected to reach the Russian border in about two weeks.
China's biggest oil company, which owns the chemical plant upriver in Jilin where the explosion occurred 12 days ago, has apologised.
Zeng Yukang, deputy general manager of China National Petroleum Corp (CNPC), expressed "sympathy and deep apologies" to the people of Harbin.
Chinese state officials blamed CNPC for the contamination and defended the government's handling of the emergency.
Zhang Lijun, of the environmental protection administration, said no decision had yet been made about whether to sue the company.
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