It was the third suicide bombing in as many days in the volatile northwest, where security forces were battling pro-Taliban Islamic militants.
Thousands of people were at the meeting in Darra Adam Khel in North West Frontier Province about 25 miles south of the provincial capital, Peshawar. The five tribes involved wanted to finalize a resolution calling for punishing anyone who sheltered or helped militants, including those of al Qaeda and the Taliban, Interior Ministry spokesman Javed Iqbal Cheema said on state-run Pakistan Television.
Alam Khan, a tribesman who attended, said a young man walked up to a group of elders and blew himself up.
"It was a huge explosion and left body parts and blood scattered on the ground," said Ramin Khan, whose left leg and face were wounded.
Dr. Hamid Afridy, the area's chief medical officer, said he counted 40 bodies, some with severed limbs and mutilated faces, at the site.
"We have dispatched more than 100 injured to ... hospitals" in Peshawar and Kohat town, he told The Associated Press. He said he feared the death toll could rise because many of the injured were in critical condition.
Pakistan Television said the suspected bomber's severed head was found.
Television footage showed blood, shoes and caps littering the bombing site - a tree-lined ground amid wheat fields that is only a few yards away from brick homes.
On Friday, a suicide bomber blew himself up at the funeral of a slain police officer in Mingora town in Swat Valley, killing more than 40 people and injuring at least 60. The next, day another suicide bombing in nearby Bajur killed one person and wounded 19 others, mostly security personnel.
By Riaz Khan