The annual monsoon is essential for irrigating crops and replenishing lakes and dams across the Indian subcontinent, but each year it also brings a wave of destruction.
The National Disaster Management Agency (NDMA) said on Friday that more than 900 people had been killed this year ― 34 of them in the past 24 hours ― as a result of the monsoon rains that began in June.
Officials say this year's floods are comparable with 2010, the worst on record, when more than 2,000 people died and nearly a fifth of the country was under water.
"I have never seen such huge flooding because of rains in my life," octogenarian farmer Rahim Bakhsh Brohi told AFP near Sukkur, in southern Sindh province.
Like thousands of others in rural Pakistan, Mr Brohi was seeking shelter beside the national motorway, because the elevated roads are among the few dry places in the endless waterlogged landscapes.
The disaster agency said more than 4.2 million people were affected by the flooding, with nearly 220,000 homes destroyed and half a million more badly damaged.
More than 809,300 hectares of cultivated crops had been wiped out in Sindh alone, the provincial disaster agency said, where many farmers live hand to mouth, season to season.
"My cotton crop that was sown on 50 acres of land is all gone," Nasrullah Mehar told AFP.
"It's a huge loss for me ... what can be done?"
Climate Change Minister Sherry Rehman, who on Wednesday called the floods "a catastrophe of epic scale", said the government had declared an emergency, and appealed for international assistance.
Pakistan is eighth on the Global Climate Risk Index, a list compiled by the environmental NGO Germanwatch of countries considered most vulnerable to extreme weather caused by climate change.
From floods to droughts
Earlier this year much of the country was in the grip of a drought and heatwave, with temperatures hitting 51ºC in Jacobabad, Sindh province.
The city is now grappling with floods that have inundated homes and swept away roads and bridges.
In Sukkur, about 75 kilometres away, residents struggled to make their way along muddy streets clogged with flood-borne debris.
"If you had come earlier the water was this high," 24-year-old student Aqeel Ahmed said, raising his hand to his chest.
Prime Minister Shehbaz Sharif cancelled a trip to Britain to oversee the flood response, and ordered the army to throw every resource into relief operations.
"I have seen from the air and the devastation can't be expressed in words," he said on state TV after visiting Sukkur.
"The towns, villages and crops are inundated by the water. I don't think this level of destruction has taken place before."
A national fundraising appeal has been launched, with Pakistan's military saying every commissioned officer would donate a month's salary towards it.
The worst-hit areas are Balochistan and Sindh in the south and west, but almost all of Pakistan has suffered this year.
Images were circulating on social media on Friday of swollen rivers in the mountainous north obliterating buildings and bridges built along their banks.
In Chaman, the western frontier town neighbouring Afghanistan, travellers had to wade through waist-high water to cross the border after a nearby dam burst, adding to the deluge brought by rain.
Pakistan Railways said nearby Quetta, capital of Balochistan province, had been cut off and train services suspended after a key bridge was damaged by a flash flood.
Most mobile networks and internet services were down in the province, with the country's telecoms authority describing it as unprecedented.