Two men were arrested after a man was killed in broad daylight on the streets of London in a suspected terror-related attack.
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said officers from the counter-terrorist unit were leading the investigation into the "shocking and horrific" Woolwich murder.
He said: "It is hard to comprehend the shocking and horrific scenes we have seen this afternoon on a busy street as Londoners went about their day as normal.
"We have launched a murder investigation, being led by the Counter Terrorism Command. Two men have been arrested in connection with that murder."
ITV News UK Editor Lucy Manning reports:
The arrests came after terrorism returned to the streets of the capital when two suspected Muslim extremists attacked a man - believed to be a serving British soldier - in the name of "Allah".
One of the alleged attackers behind the killing, near the Royal Artillery Barracks in Woolwich, was filmed wielding a bloodied meat cleaver, saying: "We swear by almighty Allah we will never stop fighting you."
In the chilling footage the suspected killer apparently explains his terrifying actions.
"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."
It was reported that a female Cub Scout leader confronted the attackers and warned them: "It is only you versus many people, you are going to lose."
The Daily Telegraph reported that Ingrid Loyau-Kennett, 48, a mother of two from Cornwall, said one of the attackers told her: "We want to start a war in London tonight".