At least 67 people have been killed and another 100 injured after a suicide bomb packed with ball bearings tore through a crowded state-run hospital in Quetta, the capital of a region of Pakistan ravaged by insurgency and sectarian tensions.

Witnesses described horrific scenes as victims cried out for help, and survivors scrambled to flee past bodies lying scattered in the smoke-filled corridors .

The blast on Monday appeared to have been a deliberate attempt to target some of Balochistan province’s most prominent lawyers, who had gathered at the hospital’s emergency department to mourn a colleague who [was?] shot dead earlier in the day in a separate incident. Bilal Kasi, the president of the Balochistan bar association, was targeted by two gunmen as he left his home for work.

Both Islamic State and Jamat-ul-Ahrar, an offshoot of the Pakistani Taliban, claimed responsibility for both attacks.

Victims of the hospital bombing included relatives of Kasi and at least two journalists who had gone to the hospital to cover his death.

One lawyer, Riaz Ahmed, 74, had gone to the hospital with an assistant and his nephew to find out what had happened to Kasi when the blast occurred. “I was near a wall and I fell down,” he told the Guardian by phone. “The wall protected me. My assistant and my nephew are injured and being treated. It was a miracle that I survived.

“There was chaos after the blast, with body parts lying everywhere. It is hard to recognise the victims. Many of the injured are quite critical.”

Another lawyer, Jameel Ahmed, 48, said he headed for the hospital as soon as he heard that Kasi, a friend, had been shot dead “I heard a loud blast, then a colleague of mine called me from hospital,” he said. “I have lost so many friends in a single attack. This is horrific. Many of those who have died were with me last night. We had dinner together and now they are all dead.

“I think the terrorists planned it so – they knew that lawyers would gather in large numbers at the hospital after the killing of Kasi, and so they sent a bomber to attack.”

Monday’s blast was the latest in a string of vicious attacks against lawyers in Balochistan, which is located in south-western Pakistan near the border with Afghanistan. It has become a breeding ground of ethnic separatist insurgency, where the minority Shia and Hazra community have been targeted in the course of past decade. Two other lawers, Jahanzeb Alvi and Amanullah Achakzai, were killed in August and in June respectively by unknown gunmen in the same city.

The Quetta hospital attack was the second deadliest in Pakistan this year. In March, on Easter Sunday, 75 people were killed in a bombing targeting Christians near a bustling park in Lahore.

Attackers have previously used similar tactics to those seen on Monday to kill people mourning at a hospital. In 2012, an attack outside a Karachi hospital killed 13 people who had gathered for a victim of a separate attack on the same day.

In an emailed statement to journalists, Jamat-ul-Ahrar said the the group “will continue attacks in Pakistan till the establishment of Islamic system in the country”. Jamat-ul-Ahrar, is a breakaway faction of the Tehreek-e-Taliban Pakistan (TTP), and is based out of the tribal areas near Pakistan and Afghanistan border. Earlier in the day, Balochistan’s chief minister, Sanaullah Zehri, had blamed Indian intelligence for being behind the attack.

The Isis-affiliated Amaq “news agency” also issued a statement claiming responsibility for the attack.

Pakistan’s prime minister, Nawaz Sharif, condemned the bombing, expressing “deep grief and anguish over the loss of precious human lives” before he arrived in Quetta to visit the hospital.

“No one will be allowed to disturb peace in the province that has been restored due to countless sacrifices of security forces, police and the people,” he said in a statement.

“This is an absolutely senseless targeting of dozens of people, including patients and mourners,” said Champa Patel, Amnesty International’s director for south Asia. “It has led to a devastating loss of life, and is an example of the string of attacks in recent years in Pakistan on schools, hospitals and other ‘soft targets’, which must cease immediately.”

She added: “A full, independent and transparent investigation must be carried out into how and why this bombing took place, and whoever is responsible should be brought to justice as soon as possible in fair trials, without recourse to the death penalty.”

According to the Express Tribune, a major English-language Pakistani newspaper, a spokesperson for the Balochistan regional government said that the attack following Kasi’s killing appeared to have been “pre-planned”.

A spokesperson for the Pakistan army said on Twitter that the attack was aimed at undermining security in Balochistan in the light of China-Pakistan economic cooperation in the region.

The Quetta bomb disposal squad has issued a statement saying at least eight kilograms of explosives were used along with ball bearings, to inflict maximum damage. The official death toll stood at 67, while more than 100 were injured. Facebook activated its safety check in the aftermath of the attack, enabling users to notify friends about their situation.

A journalist with the Pakistani television network Aaj News, Shehzad Ahmed, and Dawn’s Mehmood Khan were among the victims. Dawn said in an article published on its website that Khan had risen to become a journalist after first working as a security guard at the media organisation.

An AFP journalist who was approximately 20 metres away from the blast escaped injury. Describing the aftermath of the attack, he said: “There were pools and pools of blood around and pieces of human bodies and flesh ... people were beating their heads, crying and mourning. They were in shock and grief.”

Lawyers around Pakistan have announced a country-wide strike, and will not be attending courts on Tuesday. Pakistan’s chief of justice, Anwar Zaheer Jamali, also issued a statement condemning the attack.

Taha Siddiqui in Islamabad and Saeed Kamali Dehghan