ISLAMABAD, Pakistan — A blast detonated by a suicide bomber ripped through a Shiite protest, the second such attack in three days, in the southwestern Pakistani city of Quetta on Friday, killing at least 53 people, police and rescue officials said. At least 100 people were wounded.

Militants are intensifying their violent attacks to pressure a government that has been struggling to cope with devastating floods, described as the worst in the country’s history.

The attack was aimed at a procession of Shiite Muslims who were part of nationwide marches to observe Al Quds Day, an annual protest to express solidarity with Palestinians and condemn Israel.

It came just two days after three suicide bombers struck a Shiite procession in the eastern city of Lahore, killing 31 people and setting off violent demonstrations by infuriated survivors.

On Friday, the demonstrators were passing through a commercial neighborhood of Quetta when the blast occurred.

Shiites are a minority in the country and Sunnis hold a majority. Extremist Sunni militant groups often make religious minorities their targets.

The Pakistani news network GEO broadcast a video that showed people falling to the ground and scrambling for cover immediately after the explosion. A few people were shown firing into the smoke-filled air. The explosion led to further protests as mourners went on a rampage, clashing with the police and setting fire to vehicles and motorcycles parked nearby. Some armed Shiite men, who were accompanying the procession as private security, fired on the police, and officers fired in the air to disperse the protesters.

The wounded were taken to hospitals for treatment.

Pakistan’s interior minister, Rehman Malik, had urged religious scholars on Thursday to scale down their protest rallies, given the precarious security conditions in the country. Mr. Malik said militants were planning to take aim at Shiites to inflame sectarian tensions in the country.

Religious leaders, however, refused to take this advice. Instead, they asked Mr. Malik to resign and urged the government to provide better security for the rallies and processions.

A Shiite leader, Allama Syed Qamar Haider Zaidi, accused the government of “playing the role of a silent spectator.”

“If the government can’t provide protection to the people, then there is no justification for it to remain in power,” Mr. Zaidi said in a statement.

The United States Embassy in Islamabad condemned the recent attacks. “The deliberate targeting of innocent Pakistanis and worshipers during the holy month of Ramadan, and while the country is recovering from the terrible monsoon flooding, makes these acts particularly vicious,” it said in a statement. “Moreover, attacks on minority communities are an attack on the foundation of democracy itself.”

The floods have complicated the fight against militants. Islamic charities, some suspected of having links to militants, were quicker to deliver relief to flood victims. United Nations officials on Friday put the number of people affected by the flooding at 18.7 million. Maurizio Giuliano, a United Nations spokesman in Islamabad, said that flooding in the southern Sindh Province was not yet over.