US Secretary of State Hillary Clinton and other top foreign ministers were expected to attend.
On Friday, the international community appealed for a record two billion dollars in aid for the flood victims.
The Pakistan floods are “the worst natural disaster the United Nations has responded to in its 65-year history,” UN Secretary General Ban Ki-moon said at the launch of the appeal.
The floods caused by weeks of torrential rain have left less than 2,000 dead, according to an official toll, but the UN said the massive surge has exposed more than 20 million people to homelessness, malnutrition, risks of epidemics and loss of livelihood.
“We simply cannot stand by and watch the immense suffering in a disaster of this scale,” said Valerie Amos, the UN under secretary general for humanitarian affairs and emergency relief coordinator, announcing the mega-appeal.
The UN is seeking to focus more international attention on the floods, which emergency officials have compared to the Haiti earthquake and 2004 Asian tsunami even though the death toll is significantly lower.
The 2,006,525,183 dollars requested by 15 UN bodies will be used to help 14 million people over the next 12 months, the Office for Coordination of Humanitarian Affairs (OCHA) said.
The previous record emergency appeal made by the UN was the 1.5 billion dollars sought after the Haiti earthquake in January.
Eleven billion dollars has been sought in humanitarian appeals this year, which the UN said was the most since they started in 1991.
India was among the early contributors to the new call for cash for its neighbour and former arch-rival. It handed over a cheque for 25 million dollars at the UN headquarters.
The UN launched an appeal for 460 million dollars for Pakistan on August 11 and this is now 80 percent funded, officials said. The new appeal includes this sum.
The UN said money was needed to buy food, set up emergency camps, rebuild agriculture and villages which have seen drinking water and sanitation wiped out.
Agencies have warned of a looming health crisis in Pakistan with 709,000 cases of acute diarrhea, almost one million cases of skin disease, more than 800,000 cases of acute respiratory infections and hundreds of thousands of cases of malaria and dengue fever that are spread by mosquitoes.
The flood water is still moving from the north of Pakistan to southern provinces causing huge new emergencies.
“Yesterday, new breeches of the embankments of Manchhar Lake in Sindh flooded more villages. Millions of people have lost everything. Our task is to give people the help they need,” said Amos.
The floods have affected more than 10 percent of Pakistan’s population spread over an area bigger than 160,000 square kilometres (62,000 square miles). Some 1.9 million homes have been damaged or destroyed.
OCHA said there was “immense” damage that may take years to put right.
“Farmers who lost their crops and who are not able to plant their fields by November are likely to remain dependent on aid until well into 2012. Hundreds of thousands more lost their shops or other small businesses.”
“In these difficult financial times, countries have been extremely generous in helping those in need around the world, contributing over five billion dollars to appeals this year,” said Amos.
“But more is now needed. The government and the people of Pakistan have already done much to help families affected by these floods. We must also do our part — we simply cannot stand by and watch the immense suffering in a disaster of this scale,” she added.
The International Monetary Fund said on Wednesday it would give Pakistan a 451-million-dollar loan to help the flood recovery.