A teacher has been stabbed to death in front of pupils inside a school in Leeds.
Spanish teacher Anne Maguire, 61, was taken to hospital following the attack but later pronounced dead.
A 15-year-old boy was detained by teaching staff at Corpus Christi Catholic College and later arrested.
A pupil at the scene told the BBC that children were seen screaming and running from a classroom following the attack.
Ch Supt Paul Money of West Yorkshire Police said the victim received a number of stab wounds and a knife was recovered from the scene.
He said the 15-year-old pupil was detained by other teaching staff and arrested in connection with the stabbing.
“The incident itself was over very quickly and no other pupils or staff were threatened or injured. However this has clearly been a very traumatic situation for those involved,” Mr Money said.
He said the attack was an “unprecedented event” and schools in Leeds were “generally very safe places” to work and study.
Staff members were “in shock” and Mrs Maguire’s death would “understandably have a massive impact” on her family and the school, he added.
Downing Street said the prime minister’s thoughts were with the victim’s family.
The BBC’s education correspondent Sean Coughlan said the incident would bring back memories of Philip Lawrence, a head teacher who was killed outside a school in Maida Vale, London, in 1995.
Mr Lawrence was murdered when he went to help a pupil who was being attacked.
The call to police in Leeds came from the ambulance service shortly before 12:00 BST.
The BBC’s Danny Savage said the school was on a large estate to the east of the city, well known to residents.
“It’s regarded as a good school in what in the past could be regarded as a problematic area overall,” he said.
The Neville Road college has 950 pupils and according to its website retains “traditional values”, describing these as “a strong Christian ethos, a broad and varied curriculum, good teaching and learning, excellent discipline and outstanding pastoral care”.
Tom Riordan, chief executive of Leeds City Council, said: “We are aware of the serious incident at Corpus Christi Catholic College this morning and our thoughts are with all those concerned.
“We are working with the school to support pupils and staff at this very upsetting time.”
Mr Riordan said the authority was “working closely with the police and helping with their investigation”.
In a statement on the council’s website, the school said it would be open as normal on Tuesday.
It added: “Our school community is in shock today. Our thoughts and prayers are with the family.”
Christine Blower, general secretary of the National Union of Teachers (NUT), said the fatal stabbing was “a truly awful thing to have happened to a teacher in the course of her work to educate the next generation”.
“Appalling events like this are thankfully very rare indeed but the death of any teacher in her place of work, which should be a place of safety, is devastating.
“Our condolences go to the family and friends of this teacher, and to students and colleagues at the school.”
She said the NUT would provide help and assistance to the college and its community.
Hilary Benn, MP for Leeds Central, the constituency in which the college is located, said the killing was “profoundly saddening”.
“It is not representative of the college or the community that surrounds it,” he said.
While “shocking, terrible” attacks like this were rare, that would be “no comfort” to the dead teacher’s family, Mr Benn added.
Pupils have been paying tribute to the teacher on Twitter.
One said Mrs Maguire had “made our Year 11 best time of our life, what an amazing woman RIP”.
Another said: “I actually feel sick… RIP to probably the best teacher ever.”