Two boys, aged 12 and 13, have been rushed to hospital after being stabbed at a school in north London.
An urgent manhunt has been launched for the suspect - who police believe is a teenage boy.
The attack unfolded at Kingsbury High School on Bacon Lane in Brent at around 12.40pm today.
It is understood that the two victims are in life-threatening condition. One of them has been taken to a major trauma centre as a priority, the London Ambulance Service said.
The Daily Mail understands the two stabbings were on the school premises, but it is not known whether they were in a classroom.
Photographs from the scene show a number of emergency vehicles swarmed around the entrance to the school.
In a post on their website, Kingsbury High School said there had been a 'serious incident' at the site.
Staff have spoken directly to the parents and carers of the students involved, adding that the situation is now 'under control'.
The school was on lockdown earlier on Tuesday with no one able to enter or leave the grounds.
Parents were seen crowding around the gates to the school following the stabbing.
One mother of a Year 8 student told the Brent and Kilburn Times: 'I've been panicking, a lot of kids don't have their phones on them so I couldn't get through.
'They had to leave their bags and stuff.
'At least I have her now. It's awful - so awful - what happened, that someone would even bring a knife to a school.
'My daughter said that a fire alarm went off when it happened.'
Kingsbury High School is an academy school serving children aged 11 to 18, with both upper and lower sites.
According to the latest Department for Education information, it has 1,997 pupils.
It traces its origins to Kingsbury County School, which opened in 1925 in a building which had been the office of the former Aircraft Manufacturing Company. It moved premises in 1932.
Alumni include Wham's George Michael, jazz musician Courtney Pine, Floyd Steadman, the first black captain of the Saracens rugby team, and writer Ekow Eshun.
The London Ambulance Service said they were called at 12.41pm to reports of a stabbing on Bacon Lane.
They added: 'We sent resources to the scene including ambulance crews, incident response officers, an advanced paramedic, a paramedic from our tactical response unit and a clinical team manager.
'We also dispatched a trauma team in a car from London's Air Ambulance.
'We treated two patients in total. We took one patient to hospital and the other as a priority to a major trauma centre.'
Detective Chief Superintendent Luke Williams, who leads policing in north west London, said: 'We recognise that this incident will cause considerable concern within the community.
'We want to reassure local students, parents and local residents that we have deployed significant resources to the area and are doing everything we can to locate the suspect.
'Our thoughts are with the injured boys and I want to thank the paramedics and doctors who are providing them both with care.
'We'll provide further updates when we can.'
A spokesperson for the Met Police said the force believes the suspect is a teenage boy.
'Urgent enquiries are ongoing to locate him,' they added.
A spokesperson for Kingsbury High School said: 'We want to make you aware that there has been a serious incident at Kingsbury High School today.
'We are working closely with the relevant authorities and following all necessary procedures.
'The situation is now under control, and we have already spoken directly with the parents and carers of the students involved.
'We understand that this will be concerning. At present, it is not possible to enter or leave the school site while the response continues.
'We will provide further updates as soon as we are able to share confirmed information.'