Prime Minister Nawaz Sharif called for security forces to "decimate" terrorists after the blast at the Civil Hospital in the Balochistan provincial capital Quetta, which also wounded more than 100 people.
Video footage showed bodies strewn on the ground among pools of blood and shattered glass as shocked survivors cried and tried to comfort one another.
The bomber struck a crowd of some 200 people that had gathered to mourn the death of a senior local lawyer who had been assassinated earlier in the day.
Many of the victims were clad in the black suits and ties traditionally worn by Pakistani lawyers.
People shift an injured lawyer at the scene of a bomb blast in restive Quetta, Pakistan, 08 August 2016 Source: AAP
An AFP journalist was about 20 metres (70 feet) away when the bomb went off.
"There were huge black clouds and dirt," he said. "I ran back to the place and saw dead bodies scattered everywhere and many injured people crying.
"There were pools and pools of blood around and pieces of human bodies and flesh."
Nurses and lawyers wept as medics from inside the hospital rushed out to help dozens of injured, he said.
Pervez Masi, who was injured by pieces of flying glass, said the blast was so powerful that "we didn't know what had happened".
"So many friends were martyred," he said. "Whoever is doing this is not human, he is a beast and has no humanity."
IS, Taliban claim attack
Bomb disposal unit chief Abdul Razzaq told AFP it was a suicide attack. "The bomber had strapped some eight kilograms (18 pounds) of explosives packed with ball bearings and shrapnel on his body," he said.
The head of the provincial health department, doctor Masood Nausherwani, gave a death toll of 70, with 112 injured.
A faction of the Pakistani Taliban, Jamaat-ul-Ahrar, claimed responsibility for both the assassination of the lawyer and the blast at the hospital.
A spokesman vowed it would carry out more attacks "until the imposition of an Islamic system in Pakistan".
Hours later, the Islamic State group also said it was behind the blast, which it claimed killed 200 people, the SITE monitoring agency reported.
Middle Eastern jihadist group IS has been scrabbling for purchase in Pakistan, largely due to competition from well established extremist groups such as the Taliban.
People shift bodies of lawyers at the scene of a bomb blast in restive Quetta, Pakistan, 08 August 2016. Source: AAP
Sharif, who flew to Quetta just after the attack, said "all state security institutions must respond with full might to decimate these terrorists".
The bombing targeted a crowd made up mainly of lawyers and journalists who had gone to the hospital to mourn the president of the Balochistan Bar Association, who was shot dead on Monday.
Bilal Anwar Kasi was attacked by two unidentified gunmen as he left his home for work.
'Particularly appalling'
UN Secretary-General Ban Ki-moon condemned the attack, saying it was "particularly appalling" that it targeted mourners at a hospital.
France's President Francois Hollande denounced the "abominable act", while the European Union said there was "no justification for such acts of terrorism".
The US joined the chorus of condemnation, which the State Department said had targeted "two of the most important pillars of every democracy" -- the judiciary and the media.
Pakistan is grimly accustomed to atrocities after a nearly decade-long insurgency.
Activists of Pakistan's civil society light candles to pay tribute to the victims of bombing, in Peshawar, Pakistan, Monday, Aug. 8, 2016 Source: AAP
A military operation targeting insurgents was stepped up last year and saw the death toll from militant attacks fall to its lowest since the formation in 2007 of the umbrella Tehreek-e-Taliban Pakistan (TTP).
But analysts have warned the group is still able to carry out major attacks.
Jamaat-ul-Ahrar, part of the TTP, also said it was behind the deadliest attack in Pakistan so far this year, a bombing in a crowded Lahore park that killed 75 people on Easter Sunday.
The group has claimed responsibility for other suicide blasts, attacks on teams carrying out polio vaccinations and called for jihad in Myanmar.
Balochistan, which borders Iran and Afghanistan, has major oil and gas resources, but is afflicted by Islamist militancy, sectarian violence between Sunni and Shiite Muslims and a separatist insurgency.
Pakistani hospitals have been targeted by militants before, with a bomb killing 13 at a Karachi hospital in 2010.
