Malala Yusafzai, a 14-year-old education rights activist, has been shot and injured while on her way home from school in Mingora, the main town in the Swat Valley region of northwest Pakistan.
She is being treated at Peshawar's Combined Military Hospital, where a bullet has been removed from her skull. She remains in critical condition, family members told Al Jazeera.
Ahmed Shah Yusafzai, Malala's uncle, said there was "strict security inside and outside the hospital", after the Pakistani Taliban claimed responsibility for the attack.
Pakistan's national airline has placed an air ambulance on standby to take Yusafzai abroad for treatment if needed, government sources have revealed, but officials are wary of lengthy travel times given her unstable condition.
Yusafzai was with one other girl, taking a school van home following an examination at the Khushal public school, witnesses told Al Jazeera of the shooting.
Unidentified men stopped the vehicle, asking if it was the transport from Khushal school. When told that it was, one man asked: "Where is Malala?"
As she was identified, the assailant reportedly drew a pistol and shot Yusafzai in the head and the neck. Another girl on the bus was also wounded.
"The man started firing a handgun [...] then I don't know what happened to me and found myself in hospital," said Shazia Ramazan, a schoolmate of Yusafzai who was shot in the hand.
Doctors at the Saidu Sharif Medical Complex in Mingora said the bullet penetrated Yusafzai's skull but missed her brain, leaving her out of immediate danger.
Pakistani Taliban claim
The Tehreek-e-Taliban Pakistan (TTP), also known as the Pakistani Taliban, has claimed responsibility for the attack.
Ehsanullah Ehsan, Taliban spokesman, told reporters that the group had repeatedly warning Yusafzai to stop speaking out against them.
"She is a Western-minded girl. She always speaks against us. We will target anyone who speaks against the Taliban," he said by telephone from an undisclosed location.
"We warned her several times to stop speaking against the Taliban and to stop supporting Western non-governmental organisations, and to come to the path of Islam."
President Asif Ali Zardari strongly condemned the attack, but said it would not shake Pakistan's resolve to fight insurgents or the government's determination to support women's education. Prime Minister Raja Pervaiz Ashraf called Yusafzai "a daughter of Pakistan".
Private schools in the Swat valley have shut their doors today, in protest at the attack, though government schools are open as per their normal routine. Further demonstrations against the Taliban are also expected in the Swat district later today.
The US State Department also spoke out against the shooting.
"Directing violence at children is barbaric. It's cowardly. And our hearts go out to her and the others who were wounded, as well as their families," State Department spokeswoman Victoria Nuland said in Washington.
The local chapter of the TTP, led by Maulana Fazlullah, controlled much of Swat from 2007 to 2009, but were ousted by an army offensive in July 2009.
Local reports indicate, however, that the group was only driven into the surrounding areas, rather than being wiped out, and it has since staged a resurgence.
Tuesday's shooting in broad daylight in Mingora, the main town of the valley, raises serious questions about security more than three years after the army claimed to have crushed the local Taliban.
Yusafzai rose to international prominence in 2009, after writing a diary - under a pen-name - for BBC Urdu about life under the Taliban.
She had famously stood against the armed group's attempts to stop girls from going to school, and was awarded the National Peace Award for Youth.
The international children's advocacy group KidsRights Foundation nominated her for the International Children's Peace Prize, making her the first Pakistani girl put forward for the award.
Her struggle resonated with tens of thousands of girls who were being denied an education by the Taliban and other extremist groups across northwest Pakistan, where the government has been fighting such groups since 2007.
She was 11 years old when she wrote the blog on the BBC Urdu website, which at the time was anonymous. She also featured in two New York Times documentaries.
In a 2011 BBC news report she read out an extract of her diary that gave a sense of the fear she endured under the Taliban.
"I was very much scared because the Taliban announced yesterday that girls should stop going to schools," she said.
"Today our head teacher told the school assembly that school uniform is no longer compulsory and from tomorrow onwards, girls should come in their normal dresses. Out of 27, only 11 girls attended the school today."
London-based rights group Amnesty International condemned Tuesday's "shocking act of violence" against a girl bravely fighting for an education.
"This attack highlights the extremely dangerous climate human rights activists face in northwestern Pakistan, where particularly female activists live under constant threats from the Taliban and other militant groups," it said.
Mian Iftikhar Hussain, Khyber-Pakhtunkhwa's information minister, said Yusafzai had been targeted as "an icon of peace", calling for a sweeping military offensive against all anti-state fighters in northwest Pakistan.
Asked if Malala would continue her work if she recovered, Ahmed Shah Yusafzai, her uncle, told Al Jazeera: "Yes, of course. She always raises her voice in favour of girl's education, and she was going to establish a foundation named after her name - Malala Education Foundation - and she wanted to work for those children who are not able to go to the school."
Source: Al Jazeera and agencies