Story highlights Hundreds of worshipers were gathered for the sufi ritual of Dhamal

The Islamic State Khorasan, ISIS' affiliate in Afghanistan and Pakistan, claimed attack

Islamabad, Pakistan (CNN) At least 75 people were killed in a suicide attack Thursday at Pakistan's packed Lal Shahbaz Qalandar shrine in the southern city of Sehwan in Sindh province, according to a local hospital official.

All the dead and injured were taken to the nearby 100-bed Sehwan Hospital, which was overwhelmed the sudden influx of patients, Dr. Zahid Hussain told CNN.

More 200 people were injured in the attack, and many had been transferred to bigger hospitals in other cities of Sindh province, he said.

Thousands of worshipers, including families with their children, had gathered at the more than 800-year-old shrine for the Sufi ritual of Dhamal, which involves music, chanting and prayer.

The Islamic State Khorasan, ISIS' affiliate in Afghanistan and Pakistan, claimed responsibility for the attack in a phone call to CNN.

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