Chairman of Hanoi's People's Committee Nguyen Duc Chung called on 400 environmental workers to cleaners to remove the rotting fish.
The work was completed this morning, but more than six tons of dead fish were collected for disposal.
The chairman also instructed the Department of Environment and Natural Resources to collect water samples and establish the reason for the incident.
Last year, a number of dead fish turned up in the lake after heavy rain had followed hot weather, but the quantity wasn't as high as this year.
On the morning of June 8, thousands of dead fish floated to the surface of Hoang Cau Lake of Dong Da District. Many nearby people used nets or even waded into the water to remove the fish.
Local resident Tran Van Quan told VnExpress that he has never seen such a huge number of dead fish before.
He said he thought pollution caused by rubbish was the main reason for the incident. "There are times when the lake is cleaned out in the morning, but it is flooded by rubbish by the evening," Quan said.
Another local said that the heavy rain following a string of hot days was the reason the fish died.
Dead fish float among the litter.
A local said in anger: "The lake smells very bad. If we don't clean it up now, the lake will smell even worse at night."
Dead fish washed ashore lined up on the embankment.
A worker from the Hanoi Sewerage and Drainage Company said they had been working from morning until afternoon but dead fish were continuing to rise to the surface.
More than 400 people were sent to clean up the lake from 7 p.m. on June 8 until 1 a.m. on June 9.
After collecting all the dead fish, the workers used chemicals to purify water. Over the next few days, aquatic plants will be used to soak up pollutants from the contaminated water.
At midnight, many people were still there to see what was going on.
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