The fish died last week in the River Lea, the Hackney Gazette reported. The deaths followed storms Monday as a two-week heat wave broke.
Theo Thomas, who runs the Love the Lea campaign for the environmental group Thames21, said the rain was the last straw, washing pollutants into the Lea.
"The Lea is a river at risk, is a very polluted river all year round and on Monday it was teetering on the brink," he said. "During hot weather water holds less oxygen anyway. But fish were not dying earlier in the week, it's the pollution that killed the fish."
Government fisheries officers spent Tuesday night trying to replenish the oxygen in the river and said levels were up 25 percent by Wednesday morning.
The Lea, 42 miles long, starts in the Chiltern Hills north of London and runs into the Thames at Leamouth. Love the Lea is pushing for what are described as mini-wetlands to catch pollutants before they drain into the river.
Damien Rafferty, a Thames21 volunteer, told the newspaper the Lea is "on its last gasp."
"If the gulls weren't gorging on them, there would be a slick of dead fish across the river from side to side," he said.