Three explosions, including at least two caused by rocket fire, and gunfire were heard within minutes of the three-day "jirga" opening under what had been billed as rigorous protection enforced by 12,000 security personnel.
Two blasts were heard as Karzai delivered his opening address and condemned the Taliban for bringing suffering and oppression to Afghanistan, while a third took place later about 200 metres (yards) away, said AFP reporters.
Bursts of gunfire could also be heard in the vicinity of the giant air-conditioned tent in the southeastern Kabul suburbs, they said.
Although there was no immediate claim of responsibility for the blasts, the Taliban have dismissed the jirga as a propaganda stunt and are opposed to peace talks until US-led foreign troops Afghan soil.
The Islamist militia's nine-year insurgency against Karzai's Western-backed government is now at its deadliest and the group last month vowed to unleash a new campaign of attacks on diplomats, lawmakers and foreign forces.
Karzai appealed to the jirga delegates to advise him on how to bring the poverty-stricken country, blighted by three decades of war, out of the latest conflict and encourage the Taliban to disarm.
"We need a national consultation, a peace consultation all over Afghanistan," Karzai said.
"The Afghan nation is looking at you. They await your decisions, your advice so that you can show the Afghan nation the way to reach peace, to rescue Afghanistan from this suffering and pain."
Hundreds of bearded men in tribal dress and turbans sat in rows and a portrait of Karzai hung above the blue-carpeted stage with a shield carrying the words National Consultative Peace Jirga written in Dari and Pashto.
Women delegates, who at 300 account for about 20 percent of the total, were mostly seated in a separate section as Afghanistan's conservative social mores traditionally keep women apart from men to whom they are not related.
The meeting is the third such conference uniting Afghanistan's complex mix of ethnic, tribal, religious, geographical and gender interests since the Taliban were toppled in 2001.
But critics have warned that the outcome is likely to prove limited and ordinary Afghans have been divided about the possible results.
Taliban leaders had issued a statement Tuesday condemning the jirga.
"The foreign invading forces and their surrogates utilise this consultative jirga only as a propaganda stunt and wrongly (paint) it as a representative body of the Afghans," it said.
The jirga is being held after Al-Qaeda announced the death of its number three leader and Afghanistan operations chief Mustafa Abu al-Yazid, believed to have been killed in a US drone strike in Pakistan late last month.
Karzai's Western allies, led by the United States, have expressed support for the jirga as a milestone in Afghanistan's political maturity.
Western public appetite is waning for a war that has killed almost 1,800 foreign troops and shows no sign of abating, foreign capitals would like to see progress before an international conference set for late July in Kabul.
The number of US and NATO troops will peak at 150,000 by August as part of a strategy designed to reverse Taliban momentum and boost government authority in southern Kandahar and Helmand provinces.
US President Barack Obama has said he wants to start drawing down troops from mid-2011.
After the election of a chairman and two deputies, delegates will be divided into 28 groups, each with a spokesman to present their ideas to the general forum.
The jirga is expected to end on Friday with a declaration on what steps should be taken to end the insurgency, what groups should be included in the process and how they should be approached.
The role of Afghanistan's neighbours -- particularly Iran and Pakistan -- is considered vital to the success of any peace talks.