"Our initial information shows that the suicide attack took place when guests were exiting the tent" where the meeting was being held at around 11.30 am (0700 GMT), police spokesman Hashmat Stanikzai told local broadcaster Tolo News.
Kabul's police chief Dawood Amin told media that at least four people had been killed and one wounded in the blast. Interior ministry spokesman Najib Danish said at least 12 people had been killed and wounded, but did not give a breakdown.
The attacker detonated outside the tent, known as the Loya Jirga tent, where gatherings of top clerics and government officials are often held, Stanikzai said.
A security source confirmed it had been a suicide attack.
Local media said that some 3,000 clerics had gathered at the tent for the meeting of the Ulema Council, Afghanistan's top clerics.
Earlier they issued a fatwa against the ongoing conflict in the country, terming suicide attacks a sin against Islam.
"The ongoing war in Afghanistan does not have a legal base, only Afghans are the victims of this war... (it) does not have religious, national or human value," they said, according to government officials.
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