Reaching out on her organisation's Facebook page, Malala, who actively campaigns for female rights and education, released a moving statement.
"I am heartbroken by this senseless and cold blooded act of terror in Peshawar that is unfolding before us," the 17-year-old wrote. "Innocent children in their school have no place in horror such as this."
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Pakistan-born Malala Yousafzai said that she mourned the loss of her 'brothers and sisters'
"I condemn these atrocious and cowardly acts and stand united with the government and armed forces of Pakistan whose efforts so far to address this horrific event are commendable," added Malala. "I, along with millions of others around the world, mourn these children, my brothers and sisters – but we will never be defeated."
The Nobel Prize winner from Pakistan was herself was shot in the head by Taliban gunmen two years ago, when she had boarded her school bus in the district of Swat.
Queen Rania of Jordan expressed her grief on Twitter
Malala's message was echoed by various supporters on social media, including Queen Rania of Jordan who shared a post on Twitter.
"Till when will innocent children pay the horrific price of extremism #PeshawarAttack #ThisIsNotIslam," wrote the royal and mother-of-four.
As more details and photos of the deadly attack emerged, a range of people from political leaders to celebrities expressed their grief and horror online and the term #PeshawarAttack was soon trending.
Awful awful news from Peshawar in Pakistan. It's so sad, thinking of everyone there.— Harry Styles. (@Harry_Styles) December 16, 2014
Harry Styles tweeted to his 22.9million followers: "Awful awful news from Peshawar in Pakistan. It's so sad, thinking of everyone there," while Prime Minister David Cameron wrote: "The news from Pakistan is deeply shocking. It's horrifying that children are being killed simply for going to school."
Pope Francis condemned the incident, as well as the recent siege in Sydney and car bomb attack in Yemen, as he gave his weekly general audience in St. Peter's Square, Vatican City.
"I would like to pray together with you for the victims of the inhuman terrorist acts carried out in the past few days in Australia, Pakistan and Yemen," he said.