Addressing some 1,000 delegates from around the world, Karzai said that while Afghanistan does not want to be a "burden" on its international supporters, it would require at least another 10 years of foreign help.
It is 10 years since an earlier conference on Afghanistan in Bonn, shortly after the Taliban fell from power.
Karzai hailed the progress that had been made since then, but warned that the gains remained fragile:
"The people of Afghanistan are looking to this conference for clear affirmation of commitment to make security transition and economic progress irreversible."
The countries attending the Bonn talks are willing to make that commitment, said Germany's Foreign Minister Guido Westerwelle, cited by the Telegraph.
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He said the conference sent a clear message to the people of Afghanistan:
"We will not leave you alone, you will not be abandoned. Afghanistan and its people need a clear and reliable commitment to a long-term engagement for the next decade beyond 2014."
That engagement would include civilian reconstruction, support for the Afghan security forces and financial aid, Westerwelle said. India's Foreign Minister, S.M. Krishna, called for something on the scale of a "Marshall Plan" for Afghanistan, NDTV reported.
The United States "intends to stay the course with our friends in Afghanistan," Secretary of State Hillary Clinton told the talks. She announced that the US would resume payments into a World Bank-administered trust fund for Afghanistan, which were suspended in June over concerns about Kabul Bank.
The move should free up as much as $700 million of suspended US aid, Reuters said.
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The US was also disappointed at Pakistan's absence, Clinton said, stressing that the entire region had "much to lose" if Afghanistan becomes a source of terrorism once more.
"Nobody in this room is more concerned than the United States is about getting an accurate picture of what happened" in the incident, Clinton said.
Pakistan is thought to be crucial to negotiating lasting peace with the Taliban, as President Karzai hopes to do. He has accused Pakistan of failing to back Afghanistan's attempts at peace talks.
In addition to security concerns, Afghanistan will require financing of around £4.5 billion ($7 billion) per year by 2020 in order to maintain current levels of development, the BBC said.
The Bonn talks are not expected to produce new aid pledges, US officials told Reuters; rather, delegates will outline commitments they will finalize by mid-2012.
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