A man has died and two are injured after police responded to a reported knife attack in Woolwich, south-east London.
BBC home affairs correspondent Matt Prodger said reports suggested a man had been attacked in the street by two individuals who were later shot by officers.
London Ambulance Service confirmed one man had been found dead at the scene.
It said two men had been taken to hospital, one in a serious condition.
One of the men was airlifted from the scene, and another was taken away by road ambulance
A spokesperson said an air ambulance and four crews had attended the scene.
Witnesses reported by the Press Association said two people had been shot after armed police were called to an incident in which a man was attacked with a machete-style knife.
The incident happened close to Woolwich Barracks, and local MP Nick Raynsford said the dead man was a soldier serving there but the Ministry of Defence could not confirm this.
Mr Raynsford said police have removed a "substantial number of weapons" from the scene, including knives and at least one firearm.
One witness, identified only as James, said two men had attacked another man, aged about 20, who was wearing a Help for Heroes T-shirt.
"These two guys were crazed. They were just animals. They dragged him from the pavement and dumped his body in the middle of the road and left his body there," he told LBC radio.
Thomas, a witness who contacted the BBC, said: "I got there minutes after it happened because you could hear gunshots from Woolwich High Street.
"Basically two men carried out an axe attack on a young army cadet walking along the street, by the looks of things the police responded and then shot them in front of the public, at the same time I couldn't really tell if the cadet was fatally or not hurt as police were crowded around him."
The Independent Police Complaints Commission (IPCC) confirmed it had been informed of the incident.
In a statement, the IPCC said: "Investigators have been deployed to the scene and to the post-incident process."
Scotland Yard said the incident happened in John Wilson Street, the A205, at 14:20 BST.
On its Twitter account, the Metropolitan Police helicopter team called the incident "serious".
A Scotland Yard spokesman said: "Officers have responded to an incident...
"We believe at this stage officers were called to reports of an assault."
David Dixon, a local head teacher, told the BBC News Channel he walked out of the school gates and saw a body lying in the road a short distance away.
He then heard gunshots and instructed staff to lock all the gates of his school.
The air ambulance landed in the playground and most of the children have now gone home, he said.