Shikarpur/KHAIRPUR - At least 61 people were killed and around 50 seriously injured in a powerful explosion at a crowded imambargah during Friday prayers in Shikarpur city of Sindh province.

The bombing targeting Shia community was claimed by terrorist group Jundullah, which has earlier been targeting Hazara Shias in Balochistan and carrying out subversive activities in neighbouring Iran.

In chaotic scenes that followed the blast, part of the mosque collapsed after the explosion, burying some of the wounded under rubble. Hundreds of people rushed to the scene to try to dig out trapped survivors, witness Zahid Noon said.

Police said the blast was caused either by a suicide bomber or an explosive device (6-7 kilograms) which went off at around 1:45PM when the city’s ccentral Imambargah, Karbala Mo’alla, was at its fullest with around 400 people in attendance.

"We are trying to ascertain the nature of the blast," said Shikarpur police chief Saqib Ismail Memon. "A bomb disposal squad is examining the scene (near Lakhi Gate, clock tower square)."

Sindh Health Minister Jam Mehtab Daher said that "the death toll from the attack has increased to 61". "There are 54 dead bodies in Shikarpur hospital. Seven others died in Sukkur and Larkana hospitals," the minister said. Another 46 people were wounded in the explosion, he added.

Television footage of the aftermath showed chaotic rescue scenes as people piled the wounded into cars, motorbikes and rickshaws to take them for treatment. "The area is scattered with blood and flesh and it smells of burnt meat, people are screaming at each other... it is chaos," witness Zahid Noon said. Local resident Mohammad Jehangir said he had "felt the earth move beneath my feet" as he prayed at another mosque around 1.5 kilometre away.

An official with a Shia organisation, Rahat Kazmi, said that up to 400 people were offering Friday prayers when the blast struck. MWM central leader Allama Amin Shaheedi said it was a failure of the government. The Jafria Disaster Cell (JDC) demanded that the critically wounded victims be immediately shifted to Karachi for treatment.

Locals said there were not enough ambulances and the army later sent additional vehicles to transport people to hospitals after the bombing. Sindh Information Minister Sharjeel Inam Memon told the Nation that an emergency had been imposed at all hospitals in Shikarpur and surrounding talukas and cities.

The atmosphere was tense in the city after the explosion, with shops boarded up and crowds of emotional residents massing outside hospitals. "The entire city is in lockdown and there is tension in the air. There is a heavy police presence and the Rangers are patrolling the city," said Pariyal Marri, a local resident.

Mohabbat Ali Bablani, a Shikarpur local, said four of his cousins, aged between 30 and 40, were killed in the blast while his friend had lost five children, all under 13. "My friend Nizam-ud-Din Sheikh has lost his five sons. He had taken them with him to offer prayers and all of them were killed in the attack," he said.

Jundullah, a splinter group of Pakistan's Taliban which last year pledged support for the Islamic State group based in Syria and Iraq, claimed responsibility. "Our target was the Shia mosque ... They are our enemies," said Fahad Marwat, a Jundullah spokesman. He did not elaborate.

Majlis Wahdat-e-Muslimeen, a Shia organisation, has called for a province-wide strike on Saturday in protest. The party will also observe three-day mourning.

Sain Rakhio Merani, a regional police official, said the blast was probably caused by a bomb, although TV channels quoted some residents as saying they saw a man wearing a suicide vest.

The attack came as Pakistan tries to adopt new measures to tackle extremists following a massacre of 135 children last month at an army-run school in Peshawar. The government has pledged to crack down on all militant groups, reintroduce the death penalty, set up military courts to speed up convictions and widen its military campaign in lawless tribal areas.

Shikarpur hit was the bloodiest single sectarian attack in the country since March 2013, when a car bomb in a Shia neighbourhood of Karachi killed 45. Earlier this month, six people were killed and 17 wounded by a suicide bomber outside a Shia mosque in Rawalpindi, also after Friday prayers.

Anti-Shia attacks have been increasing in recent years in Karachi and also in Quetta, Parachinar and Gilgit. A report by the US Institute of Peace this week warned that sectarian militant groups were growing in strength in rural areas of Sindh, a province which has escaped much of the worst of the violence that has wracked Pakistan over the last decade.

Staff Reporter from Islamabad adds: Prime Minister Nawaz Sharif has strongly condemned the killing of innocent people in a suicide attack in Shikarpur. He extended condolences to the bereaved families and asked authorities to provide best medical treatment to the injured.

A PM's House statement quoted the prime minister as saying the government was committed to eradicating the menace of terrorism and extremism from the country. He said steps were being taken to improve the capacity of law enforcement agencies so that they could better cope with such challenges.

PPP's Co-Chairman Asif ali Zardari and leaders of different other political parties also strongly condemned the killing of innocent people in Shikarpur. They extended condolences to the bereaved families.