The intensity of the blast at the city's main cancer hospital raised fears of an attack, but officials initially said it was the result of a car accident.
Reuters video of the aftermath showed considerable damage to the front of the hospital, with an entrance wrecked and rubble strewn over the pavement. Victims' belongings were scattered among the debris.
The blast occurred when a car driving against traffic on Cairo's Nile corniche road collided with three other cars, the interior ministry said in a statement.
It triggered a blaze that forced the partial evacuation of the National Cancer Institute, the health ministry said.
One local resident, who gave her name as Salwa, said bodies had been fused together by the explosion and it was hard to believe it was the result of a car crash.
"There was a sound of an extremely loud blast. It was no way two cars crashing. The car must have been rigged with explosives," she told Reuters Television.
Another witness, who declined to give his name, said the car's driver ran away before it exploded.
Later in the morning, investigators scoured the scene amid a heavy police presence.
Egypt's public prosecutor is investigating the incident, sources told Reuters.Israel stands by the Egyptian people in their battle against terrorism, Prime Minister Benjamin Netanyahu said Monday, responding to the car bombing in central Cairo.“We extend our heartfelt condolences to the families of the innocent victims, and wish a speedy recovery to the injured,” he said.
Egyptian security forces are waging a counterinsurgency campaign against Islamist militants, some with links to Islamic State, that is focused in the north of the Sinai Peninsula.
Attacks outside Sinai have become relatively rare, although there have been several security incidents in recent months in Giza, across the Nile from central Cairo.In May, an explosion targeting a tourist bus injured at least 12 people, mostly South African tourists, near the Giza pyramids. In December, three Vietnamese tourists and an Egyptian guide were killed, and at least 10 others injured when a roadside bomb hit their tour bus near the pyramids.