QUETTA, Pakistan -- A suicide bomber killed at least 70 people and wounded dozens 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 and doctors said.

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

The Pakistani Taliban faction Jamaat-ur-Ahrar claimed responsibility for the attack, the Reuters news agency reported. Later, the Islamic State of Iraq and Syria (ISIS) also claimed responsibility for the attack via their Amaq news agency.

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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, the chief of the province's bar association, was shot and killed by gunmen earlier on Monday as he was on his way to his office. The lawyers gathered at the Quetta Civil Hospital to express their grief as is common with public figures. Kasi was among the most outspoken lawyers in the province and was popular for campaigning for improvements in the lawyers' community.

"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.

One of the survivors described a horrifying scene, saying there were "bodies everywhere." 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 yards away from where the bombing struck, he added.

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.

Ali Zafar, the top leader of Pakistan's main lawyers' association, denounced the bombing as "an attack on justice." He said lawyers across the country will observe three days of mourning and will stay away from court appointments to express solidarity with those killed in the attack.

Prime Minister Nawaz Sharif strongly condemned the blast in Quetta and expressed his "deep grief and anguish over the loss of precious human lives" in the attack, in which several senior lawyers were also killed.

"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.

According to Reuters, "Targeted killings have become increasingly common in Quetta, the capital of a province that has seen rising violence linked to a separatist insurgency as well as sectarian tensions and rising crime."