Sindh, in the south of Pakistan, is now being described as the country’s worst-hit province, with officials saying at least 200,000 residents have fled in the last 24 hours.
In Shahdadkot, the BBC’s Jill McGivering says residents are leaving en masse to try to reach safe ground.
The makeshift 4ft mud barrier, built by the army and volunteers, was the city’s last line of defence and has now been breached in several places.
Jam Saifullah Dharejo, Sindh provincial irrigation minister, said that most people had been escorted to safety, but efforts were under way to help those still stranded.
“We are trying to save the city from the unprecedented flood,” he said.
The Pakistan government has said that the cost of rebuilding after the floods could be as high as $15bn (£10bn).
About one-tenth of the homeless have places in relief camps, the rest are trying to survive alone, without shelter or any assurance of food, she says. Aid is being provided but it is limited and in enormous demand.
Dozens more villages have been inundated and although authorities expect flood waters to drain into the Arabian Sea over the next few days, evacuees who return may find their homes and livelihoods have been washed away.
The floods began last month in Pakistan’s north-west after heavy monsoon rains and have since swept south, swamping thousands of towns and villages in Punjab and Sindh provinces.
The UN said on Friday that more helicopters were urgently needed to reach communities cut off by the water.
Experts warn of a second wave of deaths from water-borne diseases such as cholera unless flood victims have access to supplies of fresh drinking water.
Shirkat Gah, a women’s resource centre in Pakistan, has put out an appeal for funds (the Shirkat Gah website is down as I write this, but the appeal is also hosted by the Women Living Under Muslim Laws website). Please spare some money if you can.