The suicide attack rocked the Shia mosque in Lakhi Dar area of Shikarpur district, about 500 kilometres north of Karachi, when about as 400 worshippers attended the Friday prayers.
At least 61 people were killed and more than 50 injured in the attack, claimed by Taliban splinter group Jundullah.
Police said the severed head and other body parts of the suicide bomber, who carried up to 7 kg of RDX explosives, were sent for DNA tests.
Hundreds more from the community, which make up around 20 per cent of Pakistan's population, and members of civil society gathered in the provincial capital Karachi to protest the attack. The city also observed a shutdown.
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The Sindh government announced a day of mourning today, closing schools, shops and offices, with no public transport available on the roads.
Prime Minister Nawaz Sharif said the whole nation was mourning, and vowed the government's resolve to stop terrorism "at any cost."
"It is also the matter of survival of our future generations. This war is the war of the whole nation."
Sindh Chief Minister Qaim Ali Shah visited the site of the devastating explosion and announced Rs 2 million compensation to the families of the victims.
Officials said more than 8,000 suspects have been rounded up for questioning in connection with the attack.
Yesterday's bombing was the bloodiest single sectarian attack in Pakistan since March 2013, when a car bomb in Karachi killed 45.
About 1,000 Shiites have been killed in the past two years in Pakistan.
The attack came as Pakistan increased pressure on militant groups after last month's grisly Peshawar school attack that killed 150 people, mostly school children.
Since then, the government has ended a six-year moratorium on executions in terror-related cases and pledged to crack down on all militant groups.