Rescue workers and passers by in the busy commercial district dug by hand through piles of concrete, steel and wood to recover the dead or reach victims on Monday.
Ambulances sped from the scene with the injured while bulldozers and cranes worked to clear the wreckage, surrounded by a crowd of hundreds which police struggled to control.
Farid Abdulkadir, disaster response director for Kenya's Red Cross, said there were "four confirmed dead".
"There are people trapped below and some are calling for help," he said. He said that two people had been pulled out alive so far.
Herman Wabwoba, spokesman for Kenyatta National hospital, said: "There are about 70 casualties and they're still coming in." Many were in critical or serious condition, he said.
There was no immediate word on the cause of the collapse.
At least 70 people were injured,
Construction workers had been adding more floors when the
building fell, and a Reuters reporter said some of the concrete was still wet.
Patrick Otiyo, a construction worker at the site, said: "We were working and then I felt a tremor, like the one we felt the last time, and then the building just fell."
He was referring to a 6.8 magnitude earthquake that struck East Africa in December.
Otiyo, covered in dust and scrapes, escaped serious injury but was taken to the hospital.
Kenyan police appealed to "members of the public and private sector who are trained in rescue operations to rush to the scene together with their machinery to assist".