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LAHORE, Pakistan — At least 72 people were killed and more than 320 others injured when a suicide bomber targeting Christians blew himself up at a busy park in Lahore on Sunday, officials said.

"Mostly women and children are killed and injured in the blast," Lahore Police Chief Haider Ashraf said, adding that the park was busier than usual as Easter Sunday was being celebrated. "He chose a soft target and that's why he went towards women and children in the park."

Many of the victims had lined up to buy tickets for a train ride when the blast occurred. The dead included 36 kids. The Associated Press reported that 10 members of one family were killed, all Muslims.

Witness Mohammad Arshad that he heard a "very loud" explosion.

"I was standing there near the seesaw when the blast occurred," Arshad told Reuters. "As we rushed over here we saw a pool of blood and people [both dead and injured] lying here and there."

The site of the Lahore bombing was sealed off on Monday. RAHAT DAR / EPA

A splinter group of the Pakistani Taliban — Tehreek-e-Taliban Pakistan Jamaatul Ahrar (TTP-JA) — claimed responsibility for the attack.

"Members of the Christian community who were celebrating Easter today were our prime target," TTP-JA spokesman Ehsanullah Ehsan told NBC News from an undisclosed location while using an Afghan cell number.

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Related: Why It Feels Like a 'Crime' to Be Christian in Pakistan

He added: "We didn't want to kill women and children. Our targets were male members of the Christian community."

Speaking at the mortuary, rescue worker Pervez Nazir told NBC News that the dead included 36 children. Only 10 were Christian, with the other 26 being Muslim.

The State Department called the attack a "cowardly act."

A senior U.S. intelligence official in Washington said that authorities were looking into the TTP-JA's claim of responsibility.

A child injured by the suicide attack rests in a hospital in Lahore on Monday. FAROOQ NAEEM / AFP - Getty Images

Media footage showed children and women crying and screaming and rescue officials, police and bystanders carrying injured people to ambulances and private cars.

Officials said they had recovered the suicide bomber's body. ‎He appeared to be aged in his mid-to-late 20s. Police said he had carried more than 30 pounds of explosives.

Punjab Chief Minister Shabaz Sharif announced three days of mourning in the province.

Army chief Gen. Raheel Sharif promised Pakistan "will never allow these savage non-humans to over run our life and liberty."

Pakistan is a majority-Muslim state but has a Christian population of more than 2 million.

It was the deadliest attack in Pakistan since the December 2014 massacre of 134 school children at a military run academy in the city of Peshawar that prompted a major government crackdown on Islamist militancy.

Victims' relatives cry outside a hospital in Lahore on Sunday. RAHAT DAR / EPA

Photos: Pakistanis Mourn After Deadly Easter Bombing