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From COBRA meeting: Security has been tightened at Woolwich barracks and increased at all London barracks.
London's Mayor has said it is too soon to draw conclusions, but I think it is looking more and more likely that this is a terrorist attack.
We have heard from a man - an attacker, who you see in the ITV News footgage - with a knife and blood on his hands, saying "...we swear by almighty Allah, that we will never stop fighting you. Until you leave us alone, your people will never be safe".
He goes on: "...the only reason we have done this is because Muslims are dying by British soldiers every day. This British soldier is an eye-for-eye, a tooth-for-a-tooth."
There have been very distressing scenes here at Woolwich earlier today. Now we have a massive police investigation, and the Prime Minister is flying home this evening.
President Barack Obama will visit Moore, Oklahoma - the scene of the devastating tornado - on Sunday.
Spokesman Jay Carney said the president will visit "affected families, as well as thank first responders."Authorities have confirmed two infants are among the 24 people confirmed dead.
IPCC deputy chair Deborah Glass said in a statement:
The Independent Police Complaints Commission (IPCC) has begun an independent investigation in to the circumstances surrounding the shooting of two men by the Metropolitan Police Service in Woolwich south London, today Wednesday 22 May.
We were notified of an incident by the MPS at 2.50pm. This was declared an independent investigation and investigators were deployed immediately.
A Commissioner has been appointed to lead the investigation and twelve IPCC investigators are currently at the scene, viewing CCTV footage and attending the post incident procedure.
The Metropolitan Police Service is investigating the death of one man and the IPCC is investigating the shooting of two men by police officers.
– IPCC statement
We would like to reassure the Woolwich community and wider public that our investigation will be both thorough and robust.
Anyone who was in the vicinity of Wellington Street at the junction with John Wilson Street, Woolwich, is urged to contact the IPCC by email on Woolwich@ipcc.gsi.gov.uk or using freephone 0800 096 9071.
Speaking before entering an emergency COBRA meeting, London Mayor Boris Johnson described the attack as "sickening and unforgivable act of violence."
What we have got at the moment is a very large police cordon. The Army barracks are very close to where this incident happened. The eyewitness accounts that are emerging - I have to say we don't use this word lightly - are horrific.
There has been talk of a car being crashed, at the first stages. And then reports that this wasn't a short atack, it seemed to go on for a long time - some twenty minutes. The man in the Help For Heroes t-shirt was being attacked with large knives.
One of the eyewitnesses talked about him being chopped like a "piece of meat".
Then the men stood around, eyewitnesses said, and waited for the police to come, waving knives and guns before they attacked the police. I have seen some video that was shot, where you can see one of the men standing in the street, waving knives.
This is a scene that is ongoing. The police are obviously on the scene, counter-terrorist police are involved, and certainly the suggestion is that this was a terrorist attack.
Prime Minister David Cameron has said there are strong indications that the murder of a soldier in Woolwich was terror related.
Speaking in Paris, he said: "I have been briefed by the Home Secretary about this absolutely sickening attack in Woolwich in London.
"It is the most appalling crime. We are obviously urgently seeking and the police are urgently seeking the facts of the case but there are strong indications that it is a terrorist incident."
He added: "Tonight our thoughts should be with the victim and their families and friends. People across Britain. People in every community, I believe, will utterly condemn this attack. We have had these sorts of attacks before in our country and we never buckle in the face of them."
In terms of the significance of Cobra being called, I think it is a very clear indication that officials believe this now to be without doubt a terrorist incident. What we are told by Government sources is that one of the men shouted "Allahu Akbar".
Clearly that kind of information filtering back makes the Government very, very nervous indeed. We know from many previous reports, and the many court cases we have had, that the idea of harming a soldier has been a repeated facet of terrorist plans in the past.
I think the Government will want to ask itself some very critical questions about whether this is a small group of people acting alone, or whether indeed they are part of a bigger plot. That will be their immediate concern.
David Cameron is in Paris, in discussion with Francois Hollande - clearly that will be put to one side. He is listening in on the Cobra briefing that's taking place. After that we expect him to come out and give a press conference.
Quite honestly, I would be extremely surprised if he didnt immediately get on the plane home. Parliament is not sitting at the moment. But I would be surprised, given the severity and the seriousness of the incident, if it wasn't recalled pretty soon too.