Authorities in Pakistan have evacuated at least 500,000 people from at-risk areas in the south as the worst floods in living memory threatened the country's farming heartland.

The number of people affected across the country has risen to more than 4.5 million after rivers swept through villages, washed away roads and destroyed crops.

With the death toll already numbering at least 1,500 people, authorities in the densely populated southern province of Sindh warned that major floods were expected this weekend in fertile agricultural areas along the swollen Indus river.

"We are seeing a number of preparations being made across Sindh province," Al Jazeera's Imran Khan reported from the southern coastal city Karachi.

"So far 500,000 people have been evacuated. Nearly 250,000 homes have been destroyed across the provinces bordering Sindh.

"The floods are coming further south. Nobody knows whether the floodwaters will reach Karachi, but severe flood warnings have been issued."

Villagers flee

Further north in Punjab province, Al Jazeera's Kamal Hyder said people were streaming out of flooded villages, wading barefoot through water, cramming belongings onto donkey carts and into cars under heavy rain.

"This morning when people woke up they found the water encroaching upon their land and they decided to move out.

"They are complaining that they had no warning from the authorities to evacuate so that they could save their valuables and get out in time."

The floods have affected four million people [Kamal Hyder]

Officials warned that dams could burst as heavy rains continued to lash the Punjab town of Kot Addu, transforming the area into a giant lake.

"All these villages are dangerous now. We are evacuating the population. Important installations are in danger," Manzoor Sarwar, the chief of police for Muzaffargarh district, said.

Manuel Bessler, the head of the UN Office for the Co-ordination of Humanitarian Affairs in Pakistan, warned that the country was now facing "a major catastrophe". 

More than 4.5 million people are in one way or another affected by the floods.

Survivors has lashed out at authorities for failing to come to their rescue and provide better relief.

Particular scorn has been heaped on Asif Ali Zardari, the deeply unpopular president, for pressing ahead with a visit to Europe at the height of the disaster.

However, Yousuf Raza Gilani, the Pakistani prime minister, said "relief items in sufficient quantity" have been provided.

About 30,000 Pakistani soldiers are helping to rebuild bridges, deliver food and set up relief camps in the northwest, which was hit first by the disaster.

Foreign countries and the United Nations have donated millions of dollars, and US army helicopters are assisting with evacuating stranded villagers and delivering aid.

As Zardari prepared for talks on Friday in London with Prime Minister David Cameron, British charities grouped as the Disasters Emergency Committee launched an urgent appeal on TV and radio for donations.