The bomber blew himself up among devotees in the shrine of Sufi saint Lal Shahbaz Qalandar in the town of Sehwan in Sindh province, police said.

Prime Minister Nawaz Sharif has condemned the attack, which has been claimed by so-called Islamic State.

It is the deadliest in a string of recent bombings claimed by IS, the Pakistani Taliban and other militants.

The shrine, one of the country's oldest and most revered, was crowded as Thursday is considered a sacred day for Muslims to pray there.

Witnesses said the attacker struck as worshippers were performing a sacred dance. Local media said he blew himself up after throwing a grenade.

"I saw bodies everywhere. I saw bodies of women and children," one man told local media.

Another described scenes of chaos and devastation, with the shrine's courtyard "filled with thousands of people who were crying and wailing".

The Edhi Welfare Trust, which runs Pakistan's largest ambulance service, said 43 of the dead were men, nine women and 20 children.

At least 250 others were wounded, a senior police official told the BBC. The only hospital in the area was said to be overwhelmed.

The critically injured were being sent by ambulance to Jamshoro and Hyderabad, some two hours away. The military said navy helicopters capable of flying at night would be sent to airlift the critically injured.