Shikarpur, Pakistan: At least 54 people were killed in a powerful explosion at a crowded Shi'ite mosque in Pakistan during Friday prayers, the latest sectarian attack to hit the South Asian nation.

Police said the blast was caused either by a suicide bomber or an explosive device which went off when the mosque was at its fullest on Friday afternoon in the centre of Shikarpur, a city in Pakistan's southern province of Sindh.

Radical Sunni Islamist groups often target mosques frequented by minority Shi'ites, whom they see as infidels.

Saeed Ahmed Mangnejo, head of the regional civil administration, told Reuters that the death toll had reached 54.

In chaotic scenes that followed the blast, part of the mosque collapsed after the explosion, burying some of the wounded under rubble. Bystanders pulled people from the debris and piled them into cars for the journey to hospital.

Locals said there were not enough ambulances and the army later sent additional vehicles to transport people to hospitals.

The atmosphere was tense in Shikarpur after the explosion, with shops boarded up and crowds of emotional residents massing outside hospitals.

Condemning the deadly bombing today of a Shia mosque in Sindh province in Pakistan, United Nations secretary-general Ban Ki-moon said, “vicious targeting of people on account of their religious affiliation,” and called on the Pakistan authorities to step up their efforts to protect religious minorities and combat terrorism.

Shi'ites make up about a fifth of Pakistan's mainly Sunni population of around 180 million. More than 800 Shi'ites have been killed in attacks since the beginning of 2012, according to Human Rights Watch.