The UN estimated that at least 1,600 people have died in the flooding and nearly two million have been left homeless. Some 300,000 homes have been damaged or destroyed.
See the end of this article for ways you can help the people of Pakistan.
"Make no mistake, this is a major catastrophe," UN humanitarian chief John Holmes said today to diplomats from several dozen countries in launching the appeal. "The affected population is estimated to be more than 14 million -- almost one-tenth of Pakistan's population."
The U.N. Office for the Coordination of Humanitarian Affairs, which Holmes heads, said at least 6 million or 7 million flood victims require immediate humanitarian assistance including shelter, food, clean water, sanitation and medical care. Holmes said the appeal will cover the immediate relief period of up to 90 days and will be revised in 30 days to take account of the impact of continued flooding.
Abdul Malik lived in a town of 24,000 people. Now, much of it is under 12 feet of water.
Malik pleaded for help -- he said his wife, sisters and children have been trapped at his house for a week. His house has become an island like many others. He said the government and military have not helped him.
The floods have created vast islands -- one as large as the state of Delaware.
Since flooding began with the onset of monsoon season in June, more than 14 million people have been affected by the rising waters. Countless villages and farms have been inundated, crops destroyed and livestock lost.
"You can see the top of the house but not the crop," said Dil Aram Khan, a farmer in Pirpai, Nowshera district. "We cry sometimes when we come here and look at it for awhile. then after taking a long breath we satisfy ourselves that it came from God."
"Whatever we have, it has been lost in the flood," said his wife, Noreen Bibi. "We have no source of income now. My husband is too old to work doing something else and the children have been rendered jobless by the flood."
Floods Hit Suddenly; Poor Nation Unprepared
The floods hit so quickly that there are few organized shelters, few relief workers and little hope -- in one town people were found living on the highway, in a truck stop, on the side of the road underneath beds. Hundreds were living in a building that once housed a school. Government and aid agencies are overwhelmed.
"We need a combined international response, which means more funding for this operation, more relief materials, more shelter materials, more food," said the Red Cross' Patrick Fuller in an interview with the BBC. "If we don't get that, people are going to be in an even more dire situation than they are now."
Pakistani President Asif Ali Zardari and his government have been harshly criticized for their response to the disaster. Zardari defended his decision to travel abroad as the floods began, saying he helped focus international attention on the plight of the victims.
The World Answers Pakistan's Cry for Help
Aid money is coming, says the UN. The United States made the biggest pledge, $71 million. Britain said it has given $7.8 million to the U.N. children's agency, UNICEF, for water and sanitation; $7.8 million to the Pakistani Emergency Response Fund; $15 million to rebuild bridges and $1.2 million for emergency seed money to rebuild. It has also sent 2,500 tents and $7,000 for Pakistan's emergency radio broadcast program.
Other donations included $10 million from Australia and $5 million from Kuwait.
"More than one week after the initial arrival of the floods, huge areas of the country remain without power, clear water or communications," Holmes said.
How You Can Help:
The following charities are assisting with relief efforts in Pakistan. Click the links below to contribute to their efforts.
The Department of State's Pakistan Relief Fund: You can donate $10 by texting FLOOD to 27722 or give with credit card at www.state.gov.
Make a donation by credit card at the website of the U.N. High Commissioner for Refugees, or text SWAT to 50555 to donate $10.
Make a donation to CARE.
"We have a huge task in front of us," said the UN's Holmes. "The death toll has so far been relatively low compared to other major natural disasters, but the numbers affected are extraordinarily high. If we don't act fast enough, many more people could die of disease and food shortages."
The Associated Press contributed to this report.