A former director of Zimbabwe's secret police has told the BBC that he left the ruling party over the "callous" destruction of people's homes.
President Mugabe says the operations is designed to remove criminals from cities
Former Zanu-PF MP Pearson Mbalekwa contradicted President Robert Mugabe's assertions that the operation had been planned long in advance.
"If there was a plan, we wouldn't have people sleeping under trees or next to rivers," he said.
The shanty town demolitions have left 200,000 people homeless, the UN says.
Operation Murambatsvina [Drive Out Rubbish] has been condemned by teachers, doctors, church groups, the UN and the opposition.
At the weekend, Methodist bishops from Southern Africa warned of a potential genocide.
Mr Mugabe says the six-week operation is aimed at ridding urban areas of criminals.
Mr Mbalekwa, a former senior director of the Central Intelligence Organisation (CIO), was a member of Zanu-PF's senior body, the central committee, until resigning last Friday.
He said that neither the central committee nor MPs were consulted until the crackdown had already begun.
"This thing was not planned, it was done haphazardly, thereby causing a lot of suffering to people," he told the BBC's Network Africa programme.
He said he had no idea why the operation was being carried out.
"It puzzles me and it puzzles all sane people," he said.
The opposition says its urban supporters are being punished for voting against Zanu-PF in March elections.
But many of the demolitions have also been of structures built by Zanu-PF supporters on previously white-owned farms.
UN special envoy, Anna Tibaijuka, has extended by a week her visit to Zimbabwe to assess the impact of the government campaign against illegal structures and informal traders.