A suicide bomber killed at least 70 people and wounded nearly 100 more in an attack that struck a gathering of Pakistani lawyers on the grounds of a government-run hospital in the southwestern city of Quetta on Monday, police said.

Witnesses described horrifying scenes of bodies being scattered about and the wounded screaming out and crying for help.

Nearly 100 lawyers had come to the hospital in the heart of Quetta, the capital of Baluchistan province, after the body of their colleague, prominent attorney Bilal Kasi was brought there.

Kasi was shot and killed by gunmen earlier on Monday, as he was on his way to his office, and the lawyers later gathered at the Quetta Civil Hospital to express their grief.

A Pakistani lawyer shouts slogans during a demonstration to condemn the Quetta bombing that killed dozens of people and wounded many more, in Lahore, Pakistan, on Monday. (K.Chaudary/Associated Press)

"It was a suicide attack," said Zahoor Ahmed Afridi, a senior police officer. Afridi said the attacker hit shortly after Kasi's body was brought in and that it seemed the two events were connected.​

The ISIS Amaq news agency reported the Middle East-based movement was behind the atrocity. If true, it would mark an alarming development for Pakistan, long plagued by Islamist militant violence but most of it locally-based.

Jamaat-ur-Ahrar, a faction of the Islamist militant Pakistani Taliban group, earlier said it carried out the attack, although the group is believed to have claimed responsibility for bombings in the past that it was not involved in. The movement at one time swore fealty to ISIS leadership, but later switched back to the Taliban.

The motive behind the attack was unclear, but several lawyers have been targeted during a recent spate of killings in Quetta, which has a history of militant and separatist violence.

The latest victim,Kasi, was shot and killed while on his way to the city's main court complex, senior police official Nadeem Shah told Reuters. He was the president of Baluchistan Bar Association.

Mourners may have been targeted

The subsequent suicide attack appeared to target his mourners, said Anwar ul Haq Kakar, a spokesman for the Baluchistan government. "It seems it was a pre-planned attack," he said.

Canada and the United States have condemned the attack.

Foreign Affairs Minister Stéphane Dion said: "Canada is appalled by this act of violence against civilians on the grounds of a hospital. We stand with the people of Pakistan and are committed to working tirelessly in the fight against terrorism."

White House spokesman Josh Earnest said the attack appeared to target a gathering of lawyers mourning the death of a respected colleague, which makes it all the more heinous.

Ali Zafar, president of the Supreme Court Bar Association of Pakistan, told reporters in the eastern city of Lahore, "We (lawyers) have been targeted because we always raise our voice for people's rights and for democracy ... Lawyers will not just protest this attack but also prepare a long-term plan of action."

Zafar condemned the blast as "an attack on justice." He said lawyers will observe three days of mourning and will not appear in court in solidarity with their colleagues and others killed in the attack.

Horrifying scene

One of those who survived the bombing described a horrifying scene, saying there were "bodies everywhere" after the blast. Waliur Rehman said he was taking his ailing father to the emergency ward when the explosion shook the building.

The blast was so powerful that they both fell down, he said.

When he looked up, Rehman said he saw bodies of the dead and the wounding crying out for help. He was about 200 metres away from where the bombing struck, he added.

People carry an injured lawyer following a bomb blast in Quetta, Pakistan, on Monday. A powerful bomb went off on the grounds of a government-run hospital Monday, killing dozens of people, police said. (Arshad Butt/Associated Press)

Another witness, lawyer Abdul Latif, said he arrived at the hospital to express his grief over Kasi's killing. But he said he didn't know he would "see the bodies of dozens of other lawyers" killed and wounded shortly after arriving.

Sanaullah Zehri, chief minister in Baluchistan province, said both the bombing and Kasi's slaying seemed to be part of a plot to disrupt peace in the provincial capital.

Sarfraz Bugti, the provincial interior minister, denounced the attack as an "act of terrorism." A Pakistani news channel reported that one of its cameramen was also killed in the blast.

A paramedic accompanies injured lawyers following Monday's bombing. Many lawyers are reported to be among the casualties of the deadly blast. (Arshad Butt/Associated Press)

Running in panic

Local TV stations broadcast footage showing people running in panic around the hospital grounds. Afridi said most of the dead were lawyers who had gathered after Kasi's body was brought to the hospital.

Prime Minister Nawaz Sharif strongly condemned the blast and expressed his "deep grief and anguish over the loss of precious human lives" in the attack.

"No one will be allowed to disturb the peace in the province that has been restored thanks to the countless sacrifices by the security forces, police and the people of Baluchistan," he said in a statement. Sharif asked the local authorities to maintain utmost vigilance and beef up security in Quetta.

He also instructed health officials to provide the best treatment possible to those wounded in the attack.

Gen. Raheel Sharif, the powerful army chief of Pakistan, visited the Quetta Civil Hospital, and met with those wounded in the attack.

Monday's attack was the deadliest in Pakistan since an Easter Day bombing ripped through a Lahore park, killing at least 72 people. Jamaat-ur-Ahrar also claimed responsibility for the bombing