A fire damaged stalls at Uhuru market along Jogoo Road in Nairobi from about 6 pm on Monday.
Trader Weston Gisamu said it started in a stall.
"The fire brigade are here ... we hope it will be contained before it is too late," he said on phone.
The cause was not established immediately.
Traders lost their property in a similar incident at the market in 2011.
In June, 16 people died while more than 50 were injured in a fire at Nairobi's Gikomba market.
Leaders including President Uhuru Kenyatta and Governor Mike Sonko gave instructions for thorough investigations for the frequent incidents to end.
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