A suicide blast claimed by Islamist militants ripped through crowds of families celebrating Easter at a park in the city of Lahore on Sunday, killing at least 60 people and injuring an additional 300 in an attack the jihadists said had deliberately targeted Christians.

The attack was carried out by a suicide bomber in the parking lot of Gulshan e-Iqbal Park about 6:30 in the evening, transforming a joyful scene of picnicking families into a spectacle of chaos and horror. Many children were among the dead, local officials said.

A spokesman for the Jamaat ul-Ahrar militant group, which is an offshoot of the Pakistani Taliban, asserted responsibility in a telephone interview Sunday.

“It was our people who attacked the Christians in Lahore, celebrating Easter,” the spokesman, Ehsanullah Ehsan, said. “It’s our message to the government that we will carry out such attacks again until sharia [Islamic law] is imposed in the country.”

Pakistan, a country of 190 million, has suffered for years from sectarian violence and Islamist militancy, including a Taliban-led insurgency in the tribal areas on the border with Afghanistan. Christians make up about 1 percent of Pakistan’s population but have maintained a larger presence in Lahore.

In Lahore, Parveen Masih, a 30-year-old Christian woman, said she had gone to the park with her husband and kids to celebrate Easter. They were there when the bomb exploded.

“This attack was about nothing other than to sabotage our happiness,” Masih, who was wounded in the face, said in a telephone interview. “We had only a few days to celebrate, and they didn’t even let us enjoy those.”

The government of Punjab province — where the attack occurred, and which is Prime Minister Nawaz Sharif’s political stronghold — announced three days of mourning. A statement from the office of Punjab’s chief minister, Shahbaz Sharif, who is the prime minister’s brother, pledged that the culprits would be brought to trial.

“Those who targeted innocent citizens do not deserve to be called humans,” Shahbaz Sharif posted on his Twitter account. “We will hunt you down,” he said. And “make sure your terror infrastructure is dismantled completely.”

Nawaz Sharif, the prime minister, met with his security advisers after the attack, and they reached “key decisions” on how to respond, a statement from his office said.

Ehsan, the Jamaat ul-Ahrar spokesman, declared that the militants would strike again in Punjab. The group broke away from the Pakistani Taliban, or Tehrik-e-Taliban, in 2014, as a result of infighting between top commanders. Jamaat ul-Ahrar rejoined the Taliban in March 2015, but it still maintains its own faction within the group.

The top security official in the province, Haider Ashraf, said an initial forensic investigation into the attack concluded that the suicide bomber had packed more than 20 pounds of explosives in his vest. Ball bearings, typically used in bomb attacks to maximize casualties, were found at the scene, Ashraf said.

A suicide blast exploded on Easter Sunday in a crowded park in the eastern Pakistani city of Lahore claiming the lives of at least 60 people. (Erin Patrick O'Connor/The Washington Post)

“We can say it was a suicide blast, in which most of the Christian families and Muslim families who went to Gulshan-e-Iqbal Park to enjoy the holiday were targeted,” he said, adding that the attacker detonated his explosives near an area marked off for women.

Witnesses to the carnage described body parts scattered in the wake of the attack, Pakistan’s Dawn newspaper reported. Images on social media showed panic and chaos in the moments after the blast and medics ferrying the wounded away on stretchers.

In one case, four members of a single family were killed, a medic said. The only survivor was a 10-year-old boy, who was also injured.

“I was about to enter the park with my kids” when the explosion happened, said Anwar Ali, a resident of Lahore. “My kids started crying, and I held them tightly when I saw the wounded.”

In a statement on Sunday, the State Department said that the United States “stands with the people and government of Pakistan at this difficult hour.”

“Attacks like these only deepen our shared resolve to defeat terrorism around the world,” the statement said.

The government of Punjab announced on its Twitter account that it was offering free rides to those who wished to donate blood to the victims.

In Islamabad, Pakistan’s capital, the army was deployed on Sunday to the “Red Zone” area of the city to help quell unrest following a violent protest march by Muslims. Thousands of demonstrators turned out to denounce the execution last month of Mumtaz Qadri, who assassinated the former Punjab governor, Salmaan Taseer, in 2011. Taseer had spoken out against Pakistan’s blasphemy law.

Police could not halt the demonstrators, who rampaged across central Islamabad, setting buildings on fire Sunday. The Red Zone area of the capital houses a number of vital government institutions, including Parliament and the prime minister’s house.

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Cunningham reported from Kabul. Haq Nawaz Khan and Aamir Iqbal in Peshawar, Pakistan, contributed to this report.