Extra security has been deployed to military bases in London today, after a man, reported to be a serving British soldier, was beheaded in a terrorist attack.
Scotland Yard's counter-terror unit is leading the investigation into the man's murder, after he was apparently hacked to death yesterday afternoon.
Two men are being questioned over the violent machete attack. It happened close to the Royal Artillery Barracks in Woolwich, in the south-eastern part of the city.
Witnesses say the two suspects were shot by police - they are both receiving treatment in hospital.
ITV News has obtained footage of a man with bloodied hands addressing a camera on a south London street.
Warning: Some viewers may find the graphic content in this video distressing
The incident in Woolwich has been described by the Metropolitan Police Commissioner as "shocking and horrific".
Britain's Prime Minister - who's described it as a sickening and appalling attack, a "truly shocking" event - will chair a meeting of the government's emergency committee COBRA this morning.
Speaking last night - David Cameron says terrorists won't win:
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The UK's Home Secretary Theresa May has called the murder "despicable":
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One of the men suspected of involvement in the attack was filmed wielding a bloodied meat cleaver and saying: "We swear by almighty Allah we will never stop fighting you." This eye witness explains what he saw:
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Footage shown on the ITV website shows a man, speaking to the camera, saying: "We must fight them as they fight us. An eye for an eye, a tooth for a tooth."
He added: "I apologise that women have had to witness this today, but in our land our women have to see the same. You people will never be safe. Remove your government, they don't care about you."
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.
He 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.
Mayor of London Boris Johnson responded on Twitter:
This afternoon's attack in Woolwich is a sickening deluded and unforgivable act of violence. My thoughts are with the victim and his family
— Boris Johnson (@MayorofLondon) May 22, 2013
English Defence League retaliates
Meanwhile, English Defence League supporters have thrown missiles at police in Woolwich.
A group of between 75 and 100 men gathered at The Queen's Arms pub on Burrage Grove, where they sang nationalistic songs.
Sky News correspondent Alistair Bunkall, who is at the scene, said: "As soon as the EDL got into the town centre ... it became not only a lot harder for the police to try and contain anything, but also the aggression (increased) quite considerably.
"A few missiles have been thrown, glass bottles and the like. The police are trying to surround them and they've been charging them as well with batons."
It came after two men were arrested following separate attacks on mosques hours after the suspected terrorist incident in Woolwich.
A 43-year-old man is in custody on suspicion of attempted arson after reportedly walking into a mosque with a knife in Braintree, Essex.
Meanwhile police in Kent were called to reports of criminal damage at a mosque in Canterbury Street, Gillingham, at 8.40pm.
A spokesman said a man was in custody on suspicion of racially-aggravated criminal damage.