A 16-year-old schoolboy stabbed a fellow pupil to death after being teased for being overweight, it was claimed.
Bailey Gwynne, 16, died after being knifed in the stomach during an altercation with a classmate at the Cults Academy in Aberdeen.
Traumatised school friends who witnessed the attack claimed it followed a verbal altercation in which one boy had been called fat.
During the row the suspect produced a penknife and allegedly stabbed Bailey once in the stomach.
Staff at the secondary school raced to his aid and tried to stem the flow of blood from the wound, while paramedics were called.
It is understood the incident happened in a walkway section known to pupils at "the street" at around 1.30pm on Wednesday.
Police Scotland confirmed the victim, who had four younger brothers, was rushed to Aberdeen Royal Infirmary but he was declared dead around two hours later.
Another teenager, a 16-year-old boy, was detained and was helping officers with their inquiries.
Officers remain at the school amid an "extensive investigation", which is being treated as a murder inquiry.
Pupils have been offered counselling following the murder, the local council said.
Scores of fellow pupils took to social media to pay tribute to Bailey, who is understood to come from the Maryculter area of Aberdeen.
A police spokesman said: "We can confirm that a 16-year-old boy has sadly died following a serious incident at Cults Academy. Next of kin have been informed.
"An extensive investigation is underway and a 16-year-old male has been detained in relation to the incident and is assisting us with enquiries.
"We can confirm that the Cults Academy incident is being treated as a murder investigation."
Concerned parents began arriving at the school, in the west of the city, shortly after news of the incident broke - but were prevented from entering the premises, according to reports.
Speaking about the tragedy, the school's head teacher Anna Muirhead said everyone connected with the school was "devastated".
She said: "Pupils and staff are in a state of total shock. The boy involved was a very gentle, caring pupil, with a lot of friends.
"I think it's fair to say the whole school community is totally devastated. At the moment of course our thoughts are all with the boy's close family and his friends and what they must be going through at this tragic time."
The school is described on its website as a six-year comprehensive serving the Lower Deeside area. It has about 1,050 pupils.
In 2008, Cults Academy won the Sunday Times Scottish State Secondary School of the Year award.
David MacPherson, 50, a minister at the Free Church of Scotland and a father of three children who have been pupils at the school, said: "This isn't typical of Cults.
"It's just one of those things you don't think would happen in your area.
"It's quite a quiet, affluent area. I can't remember anything like this ever happening around here. Let alone of this magnitude.
"My primary concern was to make sure that my son was OK and make sure that he wasn't involved. And now our thoughts and prayers are obviously with the family at this time."
Chief inspector Graeme Mackie said: "This is a shocking and extraordinary incident for Aberdeen, and firstly our thoughts are with the family and friends of the boy who has sadly lost his life today.
"A full and thorough inquiry has been launched and there will be a police presence in the area for the foreseeable future.
"We are working closely with the education authorities and wider community as I am sure this incident will be felt not only in Aberdeen but much further afield."
The last time a pupil was stabbed to death at school was in 2003, when 14-year-old Luke Walmsley was killed by another pupil at Birkbeck College in Lincolnshire.
Alan Pennell, 16, of Grainthorpe, Lincolnshire was later detained for life with a recommendation that he serve at least 12 years.