Donors from about 80 countries attended the one-day event, the fourth major conference on Afghanistan's future since the fall of the Taleban in 2001.
Speaking at the end of the conference, French Foreign Minister Bernard Kouchner said the donors had pledged "let's say around $20bn".
Since 2001 donors have already pledged $25bn in aid for Afghanistan, although only $15bn has been handed over.
It was unclear how much of the aid pledged on Thursday was fresh money.
The US sum includes funds already approved by Congress and is due to be spent over two years, roughly in line with current US aid levels.
US First Lady Laura Bush announced the US pledge, saying Afghanistan had "reached a decisive moment" and "we must not turn our backs on this opportunity".
The UK is committing an extra $1.2bn over the next five years, with the World Bank pledging $1.1bn over the same period.
Canada pledged $600m and Japan $550m, while Germany said it would provide 420m euros ($646m) over three years.
French President Nicolas Sarkozy, who jointly chaired the meeting with UN Secretary General Ban Ki-moon, said France would "more than double" its aid, focusing on health and agriculture.
His aides said France would commit 107m euros ($165m) over the next two years.
BBC International Development correspondent David Loyn says that little progress has been made on the benchmarks set in the last major meeting in London, two years ago, where $10.5bn was pledged.
Thursday's meeting was not just about raising more money but trying to find a better way of improving the flow of aid, and building the capacity of the Afghan government to manage affairs for itself, our correspondent says.
UN envoy Kai Eide urges donors to work more closely with the Afghan government
A recent report by the World Bank was critical of the failure of donors to build the capacity of the Afghan government to manage its own affairs.
It found there was little to show for the estimated $1.6bn that has been spent on technical assistance in Afghanistan since 2002.
Almost 70% of development spending goes outside the government, it said, much of it straight back to donor countries in the pockets of consultants, which the report described as "a second civil service".
Pressure on Karzai
In 2006-2007, Afghanistan received more than $4bn of aid, which equates to seven times Afghanistan's domestic revenues.
Nationwide, the country faces rising levels of criminal violence and government corruption fuelled by the largest opium harvests in the world, as well as the Taleban insurgency.
The country has also been hit hard in recent months by rising global food prices, with major wheat shortages as Pakistan has stopped exports.
The donor conference comes amid growing international pressure on Mr Karzai, who is expected to stand for re-election in 2009, over the lack of progress in bringing stability and improved living standards to the country since he came to power in December 2004.
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