A 15-year-old girl who was stabbed to death was attacked outside a bus while on her way to school, Sky News understands.
She was repeatedly stabbed after the attacker produced a knife from his pocket in a dispute over a bag, according to a source.
A teenage boy has been arrested after the incident near the Whitgift shopping centre in Croydon, south London.
The arrested boy is believed to have been known to the girl.
A white forensic tent and a stationary number 60 bus are within the police cordon, while footage posted on social media showed police cars and ambulances surrounding the vehicle.
A woman described seeing a bus driver and another woman trying to save the girl, who was stabbed at around 8.30am.
“I was on the bus before and came off and walked back down, I saw them resuscitating her,” said the mother-of-two who gave her first name as Bridget.
“The driver was holding her and a lady. The emergency services were already here when I walked back.”
Police said the girl was pronounced dead at the scene at around 9.20am, while the boy was arrested about 25 minutes later.
Chief Superintendent Andy Brittain said: “Our immediate thoughts are with this young girl’s family who are facing the most tragic of news. Our officers are with the girl’s family to support them.
“I am in contact with the local community, who are clearly as concerned as we are about this tragic incident. I shall continue to update them throughout the day.
“My officers were immediately on scene to provide first aid and support paramedics.”
Mayor of London Sadiq Khan said he was “absolutely heartbroken” by the death.
“My thoughts and prayers are with this young girl’s family and friends, and the whole community, at this unimaginably awful time,” he said.
“An arrest has been made in connection with this incident and I am in close contact with the Met Commissioner.”
Sarah Jones, MP for Croydon Central, said: “I am heartbroken that a child has been killed on her way to school.
“I cannot imagine what her family and friends are going through at this horrific time. Thank you to police & ambulance services for trying to save her life.”
Mayor of Croydon Jason Perry said in a statement on Facebook: “I am devastated to hear that the teenage girl who was stabbed in the town centre this morning has tragically died.
“The council are in touch with the police and will fully support their investigations. Our whole community’s thoughts will be with the victim’s family and friends.
“A suspect, who is believed to have known the victim, has been located and arrested by local police officers, and I would like to thank all those who responded so quickly and professionally to the incident.”