The UK’s government is reported to be treating the assault as a suspected terrorist incident and the prime minister said it was “truly shocking”.
Home Secretary Theresa May has summoned a meeting of the government’s emergency response committee Cobra.
Local MP Nick Raynsford said the dead man was a soldier at Woolwich barracks but this has not been confirmed.
Theresa May said she had been briefed by the director general of Security Service MI5, Andrew Parker, and Metropolitan Police Commissioner Bernard Hogan-Howe on the “sickening and barbaric” attack.
“It has been confirmed to me that a man has been brutally murdered,” she said.
“Two other men were shot by armed police and they are currently receiving treatment for their injuries.”
One witness, identified only as James, said two men had attacked another man, aged about 20, who was wearing a T-shirt of military charity Help for Heroes.
“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.
James said after the “horrendous” attack, the two men, who were also in their 20s, stood around, waving knives and a gun, and asked people to take pictures of them “as if they wanted to be on TV or something”.
“They were oblivious to anything, they were more worried about having their photo taken, running up and down the road,” he said.
Another witness, named Thomas, 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.”
Downing Street said PM David Cameron, who was planning to stay in Paris overnight after meeting the French president, would be returning to the UK tonight.
Whitehall sources said it is “a fair supposition” that the incident was a terrorist incident but police have not commented publicly on the suggestion.
It appears the police view is that the attack may have been filmed and footage may exist.
According to senior Whitehall sources, the people carrying out the attack were heard to say Allahu Akbar (God is Great).
On Twitter, the Mayor of London Boris Johnson described the attack as a “sickening deluded and unforgivable act of violence”.
Nick Raynsford, the Woolwich and Greenwich MP, said he had spoken to local police and understood a number of weapons had been seized at the scene including a gun, knives, and a machete.
Scotland Yard said the initial attack happened in John Wilson Street, the A205, at 14:20 BST.
Metropolitan Police Commander Simon Letchford said the two attackers were then shot by armed officers.
He said: “Police were called to reports of an assault… where one man was being assaulted by two other men. A number of weapons were reportedly being used in the attack, and this included reports of a firearm.
“Officers including local Greenwich officers arrived at the scene, and shortly after firearms officers arrived on the scene.
“On their arrival at the scene they found a man, who was later pronounced dead….
“Two men, who we believe from early reports to have been carrying weapons, were shot by police. They have both been taken to separate London hospitals. They are receiving treatment for their injuries.
“I can understand that this incident will cause community concerns and I would like to reiterate that we are investigating the circumstances.”
Meanwhile, the Independent Police Complaints Commission (IPCC) confirmed it had been informed.
In a statement, the IPCC said: “Investigators have been deployed to the scene and to the post-incident process.”
London Ambulance Service confirmed one man had been found dead at the scene.
It said two men had been taken to hospital – one by air ambulance – and one of them was a serious condition.
David Dixon, a local head teacher, said 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, David Dixon said.
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