At least 145 people were killed, more than 100 of them children, in a siege that lasted more than eight hours before the last of the nine attackers were killed, government and medical officials said.

A spokesman for the Pakistani Taliban said his group was responsible for the attack and said it was in retaliation for the military’s offensive against militants in the North Waziristan tribal district.

Desperate parents rushed to local hospitals or gathered outside the school gates seeking news of pupils at the school, who ranged in age from about 4 to about 16. The school had about 2,500 students in all, both boys and girls, according to the Khyber-Pakhtunkhwa Province information minister, Mushtaq Ghani.

One parent, Muhammad Arshad, described his relief after his son Ehsan was rescued from the school by army commandos. “I am thankful to God for giving him a second life,” he said.

Many others, though, were less fortunate.

The militants’ assault on the school started at about 10 a.m., when the gunmen entered the Army Public School and Degree College in Peshawar, the capital of Khyber Pakhtunkhwa Province. Local news reports said the gunmen were disguised as paramilitary Frontier Corps soldiers and gained entry by scaling a wall at the rear of the main building.

The attackers then opened fire on students with guns and grenades and, in a chilling echo of the Beslan school siege in Russia in 2004, took dozens of people hostage in the school’s main auditorium, according to news reports.

Some students managed to flee. Television coverage showed panic-stricken pupils in green sweaters and blazers, the school uniform, being evacuated from the compound. Others were wounded, and were taken to the Lady Reading Hospital in the city, where other parents gathered looking for news of their children. The hospital later published a list of students known to have died; many of the dead have not yet been identified.

By late afternoon, the army said it had cleared three sections of the school compound and that troops were pushing through the remaining sections. After the last of the militants was killed, officials said, soldiers were sweeping the compound for explosives.

Prime Minister Nawaz Sharif arrived in Peshawar, where the authorities declared three days of mourning. Mr. Sharif announced an emergency meeting of all political parties in the city for Wednesday. In a statement, the foreign ministry said it was “deeply shocked” by the attack but that the government was undeterred in its fight against the Taliban.

“These terrorists are enemies of Pakistan, enemies of Islam and enemies of humanity,” the statement said.

The British prime minister, David Cameron, called the attack “deeply shocking,” said it was “horrifying that children are being killed simply for going to school.” The American ambassador to Pakistan, Richard G. Olson, said the United States “stands in solidarity with the people of Pakistan.”