Security police used water cannons and beat squatters with heavy batons after about 2,500 Sudanese refused to leave the city park they have been occupying for the past three months in protest at their treatment by the United Nations High Commission for Refugees (UNHCR).
One Sudanese refugee told The Independent that police also used tear gas during the clashes.
According to Sudanese witnesses, up to 4,500 central security police surrounded the small park late on Thursday night. After negotiations failed, the police turned water cannons on to the men, woman and children who had been living in squalid conditions inside in the camp since September. Shortly before dawn, police used tear gas before charging the camp, destroying tents and beating the protesters in an attack which lasted half an hour.
"People were crying and screaming, children were running, old people were falling to the ground," said Hussein Sheikh Eddin, 30, a refugee from Sudan's Darfur region who lived in the camp. "It was very, very strong violence by the police. The Sudanese put up no defence."
Mr Eddin said he saw women and children being beaten. He believed that "between 10 and 30" Sudanese, including children, had been killed.
The Egyptian government had earlier placed the number of dead at 10 and claimed that the Sudanese deaths were caused by a stampede.
The forced clearance was the final act of a long-running saga that has tested the patience of the Egyptian government and strained relations with the UNHCR. The Sudanese, who had fled from the south of the country and from the province of Darfur, set up the camp near the offices of the UNHCR after efforts to resettle them to other countries had stalled.
The situation reached a stalemate last month when it became apparent that no country was prepared to accept the Sudanese, many of whom had been granted refugee status. The UNHCR had earlier announced that it would no longer provide aid to those who had applied for but been refused refugee status.
UNHCR high commissioner Antonio Guterres said he was "deeply shocked and saddened" by the attack. "There is no justification for such violence and loss of life. This is a terrible tragedy and our condolences go to all the families of those who died and to the injured," he said.
A spokesman for Egypt's Interior Ministry said it had attempted to negotiate the peaceful removal of the Sudanese, but that they had not responded. "They had no legal right to stay in the park. They threw jars of water and wine over Egyptian police," he said.
The camp had become a blot on the neighbourhood of Mohandiseen, which is home to a number of diplomatic missions and Cairo's upper middle classes.
The refugees lived crammed together with no clean water and no lavatory facilities. By night they huddled under plastic sheeting, with suitcases marking their family groupings.
Sanitary conditions had deteriorated, with local residents complaining that the Sudanese were defecating and urinating in the streets. Since the camp was set up three people, including a child, have died.
As the Egyptians began to clear up the park, Mr Eddin said: "I am very, very afraid. I have nowhere to stay. I am afraid of what will happen to us now."
However, there was no sympathy from local residents. "They've all got Aids, they're filthy, they stink," Samir Mohammed said. "They should go back where they came from."