Prime Minister David Cameron has said the attacks in Australia and Pakistan showed the worldwide threat posed by the "poisonous ideology of extremist Islamic terrorism".
Addressing a committee of MPs, Mr Cameron said: "It is nothing to do with one of the world's great religion's Islam, which is a religion of peace, this is a perversion.
"But we have to recognise the scale of what we face in this country, but also as we see around the world, and we must, with our allies, use everything we have in our power to defeat it.
"That means combating terrorism. It means defeating ISIL in Iraq and Syria but above all it means asserting the freedoms that we hold to - the values of freedom, of tolerance and democracy."
The Prime Minister added: "This is going to be the struggle of our generation both here in our own country and around the world and we're going to have to show every bit of resilience".
Some 141 people were killed in the Taliban attack on a school in Pakistan today.
A military spokesman said at least 132 children were killed along with nine staff members at the attack at the school in Peshawar, Pakistan.
The Taliban attack on a school in Pakistan is a "dark, dark day for humanity", Prime Minister David Cameron has said.
Mr Cameron paid tribute to those killed in the "appalling outrage" in Peshawar, Pakistan.
More than 120 people, including over 100 children, were killed in the attack.
Speaking before the Liaison Committee, Mr Cameron said: "The scale of what has happened in Pakistan I think just simply defies belief.
"It is a dark, dark day for humanity when something on this scale happens with no justification.
"There's not a belief system in the world that can justify this sort of appalling act."
The US Secretary of State John Kerry has condemned the "brazen" murder of more than 120 people in Pakistan today.
He said the terror act "angers and shakes" the world and that the perpetrators must be bought to justice.
Taliban gunmen stormed a school in Peshawar this morning, killing more than 100 children.
President Barack Obama has released a statement condemning the "horrific" Taliban terror attack on a school in Peshawar, Pakistan, that has killed more than 120 people, including more than 100 children.
He reiterated US support for the Pakistani government's efforts to "combat terrorism and extremism".
The United States condemns in the strongest possible terms today’s horrific attack on the Army Public School in Peshawar, Pakistan.
Our hearts and prayers go out to the victims, their families, and loved ones. By targeting students and teachers in this heinous attack, terrorists have once again shown their depravity.
We stand with the people of Pakistan, and reiterate the commitment of the United States to support the Government of Pakistan in its efforts to combat terrorism and extremism and to promote peace and stability in the region.
The nine-hour siege at a Pakistan school that has left more than 120 people dead is now over, a military source told Reuters.
Pakistani soldiers have killed nine Taliban terrorists inside the school in Peshawar, after the attackers stormed the school this morning and murdered more than 100 children and more than 20 adults.
"The operation is complete," said the source, adding that seven army personnel, including two officers, were wounded in the battle.
Hasan Abdullah, a journalist in Peshawar, Pakistan, has told ITV News that the Pakistan army is clearing the area around the school that came under attack today.
Their next move will be to look at how the gunmen gained access to the military-run school, he said.
The death toll is at least 126, which include more than 100 schoolchildren.
A Taliban spokesman said that today's attack on a school in Pakistan's Peshawar district that killed more than 100 schoolchildren was "just the trailer".
Muhammad Khorasani, spokesman for the Pakistan terror group - known as Tehrik-i-Taliban - told AFP: “They [military] was always wrong about our capabilities.
"We are still able to carry out major attacks. This was just the trailer.”
He claimed that the gunmen sent to storm the school were under orders to "shoot the older students but not the children".
A sixth terrorist inside the Peshawar school has been killed by the Pakistani military, who said that operations are "closing up".
More than 100 children have been killed in the Taliban attack today.
A military source told NBC News that Taliban attackers who stormed a Peshwar school this morning poured petrol on a teacher and set them alight in front of horrified children
"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 source, who spoke on the condition of anonymity, said that there were at least 10 attackers wearing police uniforms and suicide vests.