Tribal leaders in the Pakistani region of North Waziristan have vowed revenge against the US after drones killed more than 40 people near the Afghan border.
"We are a people who wait 100 years to exact revenge. We never forgive our enemy," the elders said in a statement.
Thursday's attack has caused fury - most of the dead were tribal elders and police attending an open-air meeting.
Observers say anger over the botched drone raid may help Pakistan delay an assault on the Taliban in Waziristan.
The Pakistani military has so far resisted US pressure for such an assault. It is already fighting militants in a number of other parts of the country's north-west.
The BBC's M Ilyas Khan in Islamabad says Thursday's casualties will also add to pressure from Islamabad on the US to scale back drone strikes which regularly target Waziristan.
The area is an al-Qaeda and Taliban stronghold and a launch pad for frequent attacks on US-led forces in Afghanistan.
But the strikes are hugely unpopular in Pakistan. The latest one comes at a time of rising tension after the CIA contractor Raymond Davis was acquitted of murdering two men in Lahore.
'Just a jirga'
Thursday's drone strike is thought to have killed more civilians than any other such attack since 2006.
Officials say two drones were involved. One missile was fired at a car carrying suspected militants. Three more missiles were then fired at the moving vehicle, hitting it and the nearby tribal meeting, or jirga.
At least four militants in the vehicles were killed, local officials said. Most of the rest who died were elders, local traders and members of the tribal police.
"The world should try and find out how many of the 40-odd people killed in the drone attack were members of al-Qaeda," the elders said in their statement following the attack near North Waziristan's regional capital, Miranshah.
"It was just a jirga being held under local customs in which the prominent elders of Datta Khel sub-division, and common people were participating to resolve a dispute.
"But the Americans did not spare our elders even.
One of the elders, Malik Faridullah Wazir Khan, said he reached the scene 30 minutes after the missiles hit - four of his relatives were killed.
"The area was completely covered in blood," he told the BBC.
"There were no bodies, only body parts - hands, legs and eyes scattered around. I could not recognise anyone. People carried away the body parts in shopping bags and clothing or with bits of wood, whatever they could find."
He said 44 people died at the scene, including 13 children - one as young as seven.
On Thursday, Pakistan's army chief condemned the raid by US unmanned drones in unusually strong terms, calling it "intolerable... and in complete violation of human rights".
The Pakistani military often makes statements regretting the loss of life in such incidents, but rarely criticises the attacks themselves.
Gen Ashfaq Parvez Kayani, however, said such "acts of violence" make it harder to fight terrorism.
US missions closed
Drone strikes have stoked anti-US feeling in Pakistan.
The US embassy in Islamabad and consulates in Lahore, Karachi and Peshawar were all closed on Friday for security reasons following Thursday's attack and the release of Mr Davis.
The US does not routinely confirm that it has launched drone operations, but analysts say only American forces have the capacity to deploy such aircraft in the region.
The Pakistani authorities deny secretly supporting drone attacks. Many militants, some of them senior, have been killed in the raids, but hundreds of civilians have also died.
Pakistan has troops stationed in North Waziristan but has resisted US calls for a wider operation there. The region is a stronghold of militants fighting US-led forces in Afghanistan.
Many analysts believe at some point Pakistan's military will have to move in - if not for America's sake, then for Pakistan's. Militants attacking targets inside Pakistan also find sanctuary in North Waziristan.