US Secretary of State Condoleezza Rice has urged African leaders to speak out against forced evictions in Zimbabwe and other alleged human rights abuses.
Some children have left school after their homes were demolished
She urged the African Union to speak out over the "tragic" events.
G8 ministers in London expressed strong concern about events in Zimbabwe and called on African countries to act.
Zimbabwean police and city-dwellers were earlier urged to be careful when demolishing illegal structures after two children were crushed to death.
A state-controlled newspaper in Zimbabwe has said that two young children were crushed to death when their homes were knocked down on the outskirts of the capital, Harare.
These are the first reported deaths in a four-week crackdown, called Operation Murambatsvina [Drive out rubbish].
One of those killed was the 18-month-old son of a police officer, reports the state-run Herald newspaper.
The UN says at least 200,000 have been left homeless in the operation, which has been condemned by the G8 group.
President Robert Mugabe says the crackdown is designed to "restore sanity" in urban areas, which he says have become overrun with criminals.
The Herald also reports that seven buildings in the centre of the capital, Harare, have been closed due to overcrowding and "health risks".
"We would like to urge those demolishing illegal structures to ensure the safety of everyone," said police spokesman Wayne Bvudzijena.
Terence Munyaka, 18 months, died on Sunday from head injuries after the walls of his house fell down in the commuter town of Chitungwiza, south of Harare.
Charmaine Nyika, aged two, died on 8 June in a similar incident in Harare, the Herald says.
The police have moved across Zimbabwe's urban areas, armed with bulldozers and sledge-hammers, destroying shacks and informal markets.
Often, residents have been made to demolish the structures themselves.
Amnesty International and the Geneva-based Centre on Housing Rights and Evictions have released a statement, they say, on behalf of 200 African and international aid groups, calling on the UN and the African Union to intervene.
They urged the AU to discuss the demolitions at next month's summit in Libya.
UK Foreign Secretary Jack Straw said African nations must stop ignoring what was happening.
"If the reports are simply half true, and we believe them to be much more than half true, this is a situation of serious international concern, and no government which subscribes to human rights and democracy should allow this kind of thing effectively to go on under their noses."
No African governments have condemned the operation.
The United Nations is due to send a special envoy to Zimbabwe to investigate the demolitions.
Some residents have been made to demolish their own homes
United States State Department deputy spokesperson Adam Ereli described the crackdown as a "tragedy, crime, horror - that the government of Zimbabwe is perpetrating on its people".
The opposition says Operation Murambatsvina is intended to punish urban voters who rejected President Mugabe in March polls.
Many people are living on the streets, while others have returned to their rural homes, encouraged by the government.
Officials also want to stamp out the black market, which they blame for Zimbabwe's economic meltdown.
A Catholic priest told the Associated Press news agency that people would never forget the "insane and evil" destruction of crops being grown on waste ground, which many rely on to supplement meagre incomes.
The blitz comes as the country urgently needs to import 1.2 million tonnes of food to avoid famine.
"It is a watershed, it is the beginning of the end, but the end will be terrible," he said.