Those uprooted from their homes in Sindh have been moved to government buildings, schools and tents.
"I didn't want to leave but when the water levels got high and we were hungry and couldn't cook anything ... my brother told me we should leave," Najma Bibi said as she searched for food with her eight-year-old son.
Fresh downpours hammered the northwestern province of Khyber Pakhtunkhwa, with experts predicting at least another day of rain, adding to the misery of the millions made homeless.
"People are camped out in the middle of the road under pieces of plastic. We are trying to reach them with relief supplies," Patrick Fuller, a spokesman for the International Federation of the Red Cross and Red Crescent, told Al Jazeera.
"The Red Crescent relief teams are on the ground but the scale of the problem facing us is absolutely massive," he said.
"The numbers keep rising every day. This crisis just reels from bad to worse."
Floodwaters have roared down from as far away as Afghanistan and India through the northwest to the agricultural heartland of Punjab and on to southern Sindh along a trail more than 1,000km long.
Raging waters have spread for tens of kilometres from rivers, wiping out entire villages.
The UN special envoy dispatched to help with the flood relief effort cancelled a flight to stricken areas and Yousuf Raza Gilani, the prime minister, also postponed his trip on Saturday because of the heavy rain.
The United Nations estimates at least 1,600 people have been killed by the floods that have ravaged the largely impoverished, insurgency-hit country, sweeping away entire villages.
The flooding has threatened electricity generation plants, forcing units to shut down in a country already suffering a crippling energy crisis.
Some parts of the Punjab are under two metres of water, affecting nearly two million people, a senior crisis management official said, while in Pakistani-administered Kashmir 63 people have died and 1,000 families displaced.
"The scale of the needs is absolutely daunting," Melissa Fleming, a spokeswoman for the office of the UN High Commissioner for Refugees, said.
More than 252,000 homes are thought to have been damaged or destroyed across Pakistan and 558,000 hectares of farmland flooded, and it could be weeks before electricity is fully restored.
Pakistan's military said on Saturday it had rescued more than 100,000 people from flood-affected areas, while 568 army boats and 31 helicopters were being used for the rescue operation.
The army was also providing food and tents to the survivors, an army statement said.
However, survivors have lashed out at the authorities for failing to come to their rescue and provide better relief, piling pressure on a cash-strapped administration struggling to contain Taliban violence and an economic crisis.
Particular scorn has been heaped on Asif Ali Zardari, the president, for pressing ahead with a visit to Europe at the height of the disaster.
Police said one man was escorted from a hall in the English city of Birmingham where Zardari gave an address after a shoe was thrown at the president.
The shoe did not hit its target, police said.