Gordon Brown, the British prime minister, said the conference "marks the beginning of the transition process" that will see "the Afghan people secure and govern their own country".
He said the summit would set a target for Afghanistan to boost its military and police forces by October 2011 in an effort to "turn the tide".
Hillary Clinton, the US secretary of state, Ban Ki-moon, the UN secretary-general, and Anders Fogh Rasmussen, Nato chief, are also among those attending the talks.
Peace and reconciliation
Karzai is seeking support for a $1bn plan that would offer cash, jobs and other incentives to the Taliban and fighters in other armed groups, in an attempt to bring them back into mainstream society.
Taliban fighters have been invited to a "peace jirga", or a traditional gathering of tribal elders, expected to be held early this year, a government spokesman confirmed on Thursday.
Hamid Elmi, Karzai's deputy spokesman, said: "We are
using all kinds of possibilities - our neighbouring countries, the international community, the king of Saudi - to encourage the Taliban to come".
But Haroun Mir, deputy director for the centre for research and policy studies in Kabul, told Al Jazeera that Karzai's proposal could be hampered by plans to increase the number of foreign troops in Afghanistan.
"Karzai has been talking for a long time for reaching out to the Taliban.
"But ... the US military surge and additional Nato forces in Afghanistan in the coming months [will see] the intensity of fighting increase.
"I don't know how president Karzai could implement his own strategy of reaching out to the Taliban if there is increased fighting going on in Afghanistan," he said.
The Taliban have dismissed Karzai's plan, saying its fighters would not be swayed by financial incentives.
Al Jazeera's James Bays, reporting from London, said the policy of outreach to the Taliban indicates a "complete U-turn" from the policies of the past US administration.
However he said it is still unclear if the strategy will be successful.
Abdullah Abdullah, a former Afghan foreign minister and Karzai's challenger in the country's recent presidential election, said reconciliation should be an immediate priority but should start at the local level.
"The door has to remain open for national reconciliation," he told Al Jazeera.
"I believe it has to be a nationally debated and transparent programme under the realm of the constitution of Afghanistan."
But Abdullah Abdullah said obstacles remain to implementing such a policy.
"I don't think the Taliban at this stage are willing to enter negotiations. Also, their association with terrorist organisations, like al-Qaeda - that's the main issue at the moment - and they are working like one organisation."
Thursday's conference comes nearly a week after a meeting between Afghan government officials and members of an armed opposition group fighting alongside the Taliban.
Al Jazeera's David Chater, reporting from Kabul, said the talks were held with the group Hezb-e-Islami, in the Maldives islands, between January 23-24.
He said a Taliban leader had been due to attend the meeting but dropped out in the last minute citing health reasons.