Witnesses described how a huge blast shook the Army Public School in the northwestern city of Peshawar and gunmen went from classroom to classroom, shooting children.

The gunmen killed 141 in total, including 132 children and 9 teachers, according to the most recent reports.

"In CMH (Combined Military Hospital) there are around 60 and there are 24 dead in Lady Reading (hospital)," Pervaiz Khattak, Chief Minister of the province where Pehsawar is located, told local television channels. 

Distraught parents thronged the city's Lady Reading Hospital in the wake of the attack, weeping uncontrollably as children's bodies arrived, their school uniforms drenched in blood.

The Tehreek-e-Taliban Pakistan (TTP) claimed responsibility for the attack as retaliation for a major military offensive in the region, saying militants had been ordered to shoot older students.

"We selected the army's school for the attack because the government is targeting our families and females," said Taliban spokesman Muhammad Umar Khorasani. "We want them to feel the pain."

The Afghanistan Taliban, meanwhile, have condemned the attack. "The intentional killing of innocent people, children and women are against the basics of Islam and this criteria has to be considered by every Islamic party and government," Zabihullah Mujahid said in a statement.

The Pakistani Taliban are separate from but allied to the Afghan Taliban across the border. Both aim to overthrow their own governments and establish an Islamic state. 

According to early witness accounts, the gunmen fired indiscriminately at children and teachers. It is the bloodiest massacre the country has seen for years.

More than eight hours after militants entered the school compound, the military declared the operation to flush them out over, and said that all nine insurgents had been killed.

The attack at a military-run high school attended by at least 500 students, many of them children of army personnel, struck at the heart of Pakistan's military establishment, an assault certain to enrage the country's powerful army.

The attack began around 10.30am local time when a group of nine insurgents, reportedly in military uniforms, entered the school.

The Taliban said the gunmen had been equipped with suicide vests. Three explosions were heard inside the high school at the height of the massacre, raising fears of more casualties.

Outside, as helicopters rumbled overhead, police struggled to hold back distraught parents who were trying to break past a security cordon and get into the school.

Officials said 122 people were wounded. A local hospital said the dead and injured were aged from 10 to 20 years old.

The gunmen, who several students said communicated with each other in a foreign language, managed to slip past the school's security because they were wearing Pakistani military uniforms, local media reported.

A security official said hundreds of students and staff were in the school when the attack began, though according to the military the bulk of them were then evacuated.

Around five-and-a-half hours after the attack began, the army's chief spokesman General Asim Bajwa said the attackers had been cleared from all but one of the school's buildings.

Five militants had been killed, Bajwa said.

Prime Minister Nawaz Sharif described the attack as a "national tragedy unleashed by savages".

"These were my children. This is my loss. This is the nation's loss," he said.

Provincial Information Minister Mushtaq Ghani said the death toll had reached 130, with a similar number wounded. The toll was confirmed by another provincial minister.

Provincial Chief Minister Pervez Khattak said the attackers were wearing uniforms of the government paramilitary Frontier Corps.

Mudassar Abbas, a physics laboratory assistant at the school, said some students were celebrating at a party when the attack began.

"I saw six or seven people walking class-to-class and opening fire on children," he said.

One student said soldiers came to rescue them during a lull in the firing.

"When we were coming out of the class we saw dead bodies of our friends lying in the corridors. They were bleeding. Some were shot three times, some four times," the student said.

"The men entered the rooms one by one and started indiscriminate firing at the staff and students."

The school on Peshawar's Warsak Road is part of the Army Public Schools and Colleges System, which runs 146 schools nationwide for the children of military personnel and civilians.

Its students range in age from around 10 to 18.

The Pakistani Taliban, who are fighting to topple the government and set up a strict Islamic state, have vowed to step up attacks against Pakistani targets in response to a major army operation against the insurgents in the tribal areas.

The army said in a statement that many hostages had been evacuated but did not say how many.

"Rescue operation by troops underway. Exchange of fire continues. Bulk of student(s) and staff evacuated. Reports of some children and teachers killed by terrorist," the army said in a brief English-language statement.

Military officials at the scene said at least six armed men had entered the military-run Army Public School. About 500 students 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.

One student inside the school at the time of the attack told a private television channel: "We were in the examination hall when all of sudden firing started and our teachers told us to silently lay on the floor. We remained on the floor for an hour. There was a lot of gunfire.

"When the gunfire died down our soldiers came and guided us out."

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.

"It's a revenge attack for the army offensive in North Waziristan," he said, referring to the anti-Taliban military offensive that began in June.

A Reuters journalist at the scene heard heavy gunfire from inside the school as soldiers surrounded it. Helicopters hovered overhead and ambulances ferried wounded children to hospital.

The Lady Reading Hospital in Peshawar, a sprawling and volatile city not far from the Afghan border, earlier said the hospital had received the bodies of at least 12 people and was treating 40 wounded students and two male teachers.

"Many are in the operation theatre now in critical condition, undergoing treatment," said hospital official Ejaz Khan.

"An army doctor was visiting us teaching us about first aid when attackers came from behind our school and started firing," one student told Pakistan's Dunya Television.

"Our teachers locked the door and we ducked on the floor, but they (militants) broke down the door. Initially they fired in the air and later started killing the students, but left the hall suddenly.

"The attackers had long beards, wore shalwar kameez (traditional baggy clothes) and spoke Arabic."

The Taliban had earlier said they had sent six insurgents with suicide vests to attack the school.

The Pakistani Taliban, who are fighting to topple the government and set up a strict Islamic state, have vowed to step up attacks in response to a major army operation against the insurgents in the tribal areas.

They have targeted security forces, checkpoints, military bases and airports, but attacks on civilian targets with no logistical significance are relatively rare.

In September, 2013, dozens of people, including many children, were killed in an attack on a church, also in Peshawar, a sprawling and violent city near the Afghan border.

Pakistanis, used to almost daily militant attacks, were shocked by the scale of the massacre and the loss of so many young lives.

It recalled the 2004 siege of a school in Russia's Beslan by Chechen militants which ended in the death of more than 330 people, half of them children.

Malala condemns Taliban school attack

Nobel Peace laureate Malala Yousafzai says she's heartbroken by killing of scores of children by Taliban militants in Pakistan.

17-year-old Malala, who was shot in the head by the Taliban in 2012, says the incident is a senseless and cold blooded act of terror.

Reuters, AFP