The bomb went off as tens of thousands celebrated the Prophet Muhammad's birth in the Pakistani city of Karachi.
A suicide bomber is believed to have detonated his explosives on the wooden stage during evening prayers.
Influential Sunni Muslim leaders were among at least 47 people killed in the attack.
Fellow worshippers rushed to aid at least 80 people who were wounded.
Ambulances gathered at the scene as the city's hospitals were put on alert.
Survivors were distraught by the attack on their holy ritual in Karachi's historic Nishtar park.
But shock turned to anger as crowds began accusing the police of failing to provide adequate security.
Dozens of vehicles were torched and security forces pelted with stones as protesters took to the streets.
A petrol station was also set alight.
Paramilitary troops had to be brought in to control the situation, firing shots into the air to disperse the crowds.