An assault led by the Taliban on a Pakistan military-run school Tuesday left 141 people dead, 132 of which were children, officials say, in the worst attack to hit the country in years.

The horrific attack in Peshawar, carried out by a relatively small number of militants from the Tehreek-e-Taliban group, a Pakistani militant group trying to overthrow the government, also sent dozens of wounded flooding into local hospitals as terrified parents searched for their children.

"My son was in uniform in the morning. He is in a casket now," wailed one parent, Tahir Ali, as he came to the hospital to collect the body of his 14-year-old son Abdullah. "My son was my dream. My dream has been killed."

Pakistani military spokesman Asim Bajwa told a news conference Tuesday that 132 of the dead were children and another nine were staff members.

He said there were seven attackers, who all wore explosive vests.

A local hospital official told Reuters that the dead and wounded it had seen were between the ages of 10 and 20 years old.

As darkness fell on the area, officials said the country's military had cleared the school of militants.

"The operation is completed," said Bilal Ahmad Faizi, the head of the state-run rescue organization, speaking to reporters after leaving the school area.

The attack began in the morning hours, with about half a dozen gunmen entering the school — and shooting at random, said police officer Javed Khan. Army commandos quickly arrived at the scene and started exchanging fire with the gunmen, he said. Students wearing their green school uniforms could be seen on Pakistani television, fleeing the area.

Outside the school, two loud booms were heard coming from the scene in the early afternoon, as Pakistani troops battled with the attackers. Armored personnel carriers were deployed around the school grounds

Pakistani television showed soldiers surrounding the area and pushing people back. Ambulances streamed from the area to local hospitals.

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One of the wounded students, Abdullah Jamal, said that he was with a group of 8th, 9th and 10th graders who were getting first-aid instructions and training with a team of Pakistani Army medics when the violence began for real.

When the shooting started, Jamal, who was shot in the leg, 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.

Another student, Amir Mateen, said they locked the door from the inside when they heard the shooting, but gunmen blasted through the door anyway and began shooting.

The school is located on the edge of a military cantonment in Peshawar, but the bulk of the students are civilian. Peshawar has been the target of frequent militant attacks in the past, but has seen a relative lull recently.

The Pakistani military launched a widespread military operation in the North Waziristan tribal area in June, vowing that it would go after all militant groups that had been operating in the area. With the launch of the operation, security officials and civilians feared a backlash by militants targeted by the military but until Tuesday a widespread backlash had failed to materialize.

Pakistani Prime Minister Nawaz Sharif condemned the assault in Peshawar and rushed to the area to show his support for the victims.

The prime minister vowed that the country would not be cowed by the violence and that the military would continue with the aggressive operation.

"The fight will continue. No one should have any doubt about it," Sharif said.

At the same time, Sharif also announced a three-day countrywide mourning over the attack, Dawn.com reports.

Tuesday's attack calls into question whether the militants have been crippled by the military or will be able to regroup. This appeared to be the worst attack in Pakistan since the 2008 suicide bombing in the port city of Karachi killed 150 people.

The violence also underscored the vulnerability of Pakistani schools, which was dramatically exposed in the attack two years ago on Malala Yousafzai, a Pakistani girl shot in the head by a Taliban gunman outside her school in Swat Valley for daring to speak up about girls' rights. She survived, becoming a Nobel Prize laureate and global advocate for girls' education.

Militants also have blown up schools in the northwest.

"I am heartbroken by this senseless and cold-blooded act of terror in Peshawar that is unfolding before us," said Malala in a statement. "I, along with millions of others around the world, mourn these children, my brothers and sisters, but we will never be defeated."

The attack also drew condemnation from U.S. Ambassador to Pakistan Richard Olson.

“The United States strongly condemns senseless and inhumane attacks on innocent students and educators, and stands in solidarity with the people of Pakistan, and all who fight the menace of terrorism,” he said in a statement.

“Few have suffered more at the hands of terrorists and extremists than the people of Pakistan,” the statement added. "That is why it remains essential for the United States and Pakistan to continue to work together to secure peace and stability in the region.

The Associated Press contributed to this report.