Kenyan President William Ruto said he had deployed a team of emergency responders, including soldiers, to co-ordinate rescue efforts while offering condolences to the affected communities.
"I have also ordered that relief food from our national strategic reserves be immediately released and distributed to families affected by the floods," he said on social media.
In the industrial neighbourhood of Grogan, security guard John Lomayan, 34, looked at the body of an elderly man he recognised - a roadside egg seller - trapped beneath a car that had been washed away when the Nairobi River burst its banks.
"I saw him being carried by the water from up there," he said, gesturing up the road.
"We didn't know where he had gone. It is only now that we see him under the car".
Bus driver John Mwai recounted how he turned his bus into a rescue vehicle to move people to higher ground.
Kenya Airways said the rains had disrupted flights to Nairobi and forced some to divert to the coastal city of Mombasa.
A Reuters reporter saw three bodies pulled from underneath cars.
Some of the dead had been electrocuted by damaged power lines.
National provider Kenya Power separately said the waters had damaged equipment at a substation, listing 14 neighbourhoods that had been affected.
"So many cars, so much stuff, I don't know. Everything was just (washed away). All of the water (came) ... from that river," shocked resident Cedric Mwanza said, referring to the Nairobi River.
