A morning fire on Monday destroyed a large section of Toi market in Kibera, rendering many traders jobless.
Police said the fire started in one of the stalls before spreading fast to others.
The officers said preliminary findings indicated the fire, which destroyed property of unknown value, was caused by an electric fault.
Kilimani divisional police boss Peter Katam said they were informed of the fire at about 2am.
"No lives were lost but property was destroyed. We are investigating the incident," said Mr Katam.
READ MORE
Kidero tells CS Mbadi to respect Odinga family
Blow to DPP as court rejects bid to close Kidero's Sh58 million graft case
Church to Court: Tuju denies giving false information charge after prayers
Boost to Kibera children as Australian education partners launch community library
futile effort
Those affected have asked for help to rebuild the market.
County leaders led by Governor Evans Kidero, CORD leader Raila Odinga and area MP Ken Okoth went to the scene to assess the damage.
Edward Mwangi, one of the traders at the market, said efforts to put it out were futile because it spread very fast.
"We did not salvage anything. Our goods were burned. I have lost Sh100,000 worth of stock in shoes and other wares I had," he said.
Dr Kidero empathised with the traders and pledged to help them get back into business.
He said the Nairobi County government had already set aside Sh5 million to rebuild the market.
Toi market houses thousands of locals an is a source of livelihood for many. Traders deal in fresh food and clothes.
Meanwhile, a three-year-old child died in a fire in Kawangware, Nairobi.
Police say the child was asleep when the fire broke out.
Property of unknown value was destroyed in the incident.
The child's body was removed from the rubble long after the fire had been contained.
"We are investigating the fire with a view to establishing whether it was the result of criminal activity," said Dagoretti OCPD Rashid Mohamed.