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PESHAWAR, Pakistan — Pakistan was plunged into mourning Tuesday after Taliban militants in suicide vests laid siege to a school, massacring 132 children and 10 teachers during eight hours of sheer terror. In total, 145 people were killed, including three soldiers, officials said.

Those who survived emerged with stories of horror — of gunmen shooting indiscriminately into crowds or killing youngsters one by one.

"One of my teachers was crying, she was shot in the hand and she was crying in pain,'' Shahrukh Khan, 15, who was shot in both legs but survived, told Reuters. "One terrorist then walked up to her and started shooting her until she stopped making any sound.

"All around me my friends were lying injured and dead.''

A military source told NBC News that the attackers were wearing police uniforms and suicide vests.

"They burnt a teacher in front of the students in a classroom,” he said. “They literally set the teacher on fire with gasoline and made the kids watch."

The government of Pakistan declared three days of mourning for the lives lost.

Maj. Gen. Asim Bajwa, a military spokesman, told NBC News that at least 132 children were killed in the attack, along with 10 staff from the school — including the principal. Seven militants were killed and seven special forces soldiers were injured.

"They didn't take any hostages initially and started firing in the hall," Bajwa also told a press conference. He told NBC News that they had enough ammunition and rations to have kept up the siege for days.

At a hospital near the school, blood stained the floors. Crying relatives roamed the wards and searched operating rooms, desperately searching for their sons and daughters.

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One room at at the Central Military hospital was filled with teenagers who had bullet wounds, shrapnel embedded in their flesh and burns.

A doctor, Brig. Muhammad Waqar, said his son attends the school and he watched with dread as victim after victim was brought in

"I was waiting for him to turn up dead in an ambulance," he said. "I wanted to grab a gun and go to the school."

The Pakistani Taliban have claimed responsibility for the attack, which Pakistani officials said appeared to be aimed at the children of senior military personnel.

Uniformed militants struck shortly before 11 a.m. local time (1 a.m. ET) when about 1,000 students — in grades one through 10 — and teachers were believed to be inside.

"We were standing outside the school and firing suddenly started and there was chaos everywhere and the screams of children and teachers," said Jamshed Khan, a school bus driver.

"The gunmen entered class by class and shot some kids one by one," one student who was in the Army Public School in Peshawar at the time told local media.

As the siege continued and Pakistani security forces battled to stop the assault, five "heavy" explosions were heard from the school at around 5 a.m. ET. Bombs planted by the attackers slowed rescue efforts, a military official said, and the massacre was not declared over until after 9 a.m. ET.

Wounded student Abdullah Jamal told The Associated Press he was getting first-aid instructions and training with a team of Pakistani army medics when the violence began for real. When the shooting started, Jamal said nobody knew what was going on in the first few seconds.

"Then I saw children falling down who were crying and screaming. I also fell down. I learned later that I have got a bullet," he said, speaking from his hospital bed. He had been shot in the leg.

Hours after all the children had been removed from the school, soldiers angrily roamed the campus.

"It's interesting that they came through this graveyard," said one officer, pointing to a cemetery adjacent to the school. "It's sad. They stepped over the graves of the dead to create more death."

President Barack Obama slammed the attack and said America stands with the people of Pakistan and its government's efforts to fight terrorism.

"By targeting students and teachers in this heinous attack, terrorists have once again shown their depravity," he said in a statement.

AmbOlson Extends Condolences to Families of Army Public School Attack Victims http://t.co/5jgJf9vNcQ — US Embassy Islamabad (@usembislamabad) December 16, 2014

As the carnage played out in Peshawar, Pakistan’s military carried out 10 airstrikes in the Khyber region, between Peshawar and the Afghanistan border, based on “actionable intelligence” according to a spokesman.

The Pakistani Taliban has vowed to attack government targets as it fights off a huge army operation in the country's tribal region.

Taliban spokesman Muhammad Umar Khorasani told Reuters his group was responsible for the attack. "Our suicide bombers have entered the school, they have instructions not to harm the children, but to target the army personnel," he said.

Reuters and The Associated Press contributed to this report.