At least 61 people have been killed in a bomb blast at a Shia mosque in southern Pakistan.
Dozens were wounded in the attack after Friday prayers in Shikarpur district of Sindh province, and the death toll is expected to rise.
Sunni militants linked to the Pakistani Taliban said they carried out the attack.
Local media reports suggest that the blast could have been a suicide attack, but police are investigating.
There has been rising sectarian violence in Pakistan in recent years. Sunni militant groups have targeted the Shia minority in the past.
Attacks against religious minorities have been on the rise in the country where radical Sunni Islamist groups often target mosques frequented by Shi'ites, whom they see as infidels.
Police said it was unclear what caused the explosion during Friday prayers at the mosque in the busy centre of Shikarpur, a city in Sindh.
"We are trying to ascertain the nature of the blast," said city police chief Saqib Ismail Memon. "A bomb disposal squad is examining the scene."
Part of the mosque collapsed after the explosion, burying some of the wounded under rubble. Bystanders pulled them out and piled them into cars for the journey to hospital.
After the tragedy struck, Inspector General (IG) Sindh Police Ghulam Qadir Thebo formulated a two member committee to investigate the bombing under the leadership of Senior Superintendent of Police (SSP) Raja Umar Khattab.
SSP Khattab visited the site of the fatal blast along with SSP Mazhar Mashwani today and observed the destruction rendered by the deadly attack.
After collecting crucial evidence, SSP Khattab confirmed to Dawn that the explosion was a suicide blast.
A counter-terrorism team of officers from the Federal Investigation Agency (FIA) also proceeded to the blast site on Saturday for investigation and took statements from eye-witnesses.
Police officials said the suicide jacket had been filled with ball bearings, the use of which is aimed at magnifying the impact of the blast.
The investigation has moreover revealed that the explosive material used in the blast weighed five to seven kilograms.
The Jafria Disaster Cell (JDC) demanded that the critically wounded victims be immediately shifted to Karachi for treatment.
Local resident Mohammad Jehangir said that he had “felt the earth move beneath my feet” as he prayed at another mosque around 1.5 kilometres away.
An official with a national Shia organisation, Rahat Kazmi, said that up to 400 people were worshiping in the mosque when the blast struck.
Prime Minister Nawaz Sharif, President Mamnoon Hussain, Pakistan Tehreek-e-Insaf Chairman Imran Khan and Muttahida Qaumi Movement chief Altaf Hussain issued condemnation messages against the incident.
Speculation prevails that the explosion may have been carried out by a suicide bomber.
The incident comes as Pakistan is attempting to implement the National Action Plan to combat and root out terrorism from the country, an initiative that was set in motion after the Dec 16 attack on Peshawar’s Army Public School in December 2014.
It was the second major attack on an imambargah in the country since the beginning of 2015; the first being an attack on Rawalpindi’s Imambargah Aun Mohammad Rizvi in the garrison city’s Chatian Hatian area.
At the same time, Pakistani security forces are engaged in the North Waziristan and Khyber tribal regions — with the operation in Waziristan starting soon after a terrorist attack on Karachi’s Jinnah International Airport and the theater of war expanding into Khyber