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Two people have been arrested in connection with a murder in Woolwich today.
Metropolitan Police commissioner Sir Bernard Hogan-Howe 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.
We understand concern about the motivation and we will work tirelessly to uncover why this occurred and who was responsible. I understand people want answers, but I must stress we are in the early stages of investigations.
– Bernard Hogan-Howe
I would urge everyone to remain calm and have a measured response to today's tragic events.
In Greenwich we have met with community representatives, and extra officers remain on duty there tonight.
Across London our officers are in contact with their communities too. The shock that we all feel at what has happened must bind us together.
Around 40 members of the far-right organisation the English Defence League have run towards police in Woolwich following today's attack in the area.
Officers are now flooding the streets after the organisation called on its members to meet in the area to stage a protest.
Met Police Commissioner Bernard Hogan-Howe said two men have been arrested in connection with a murder in Woolwich today.
Labour leader Ed Miliband said the killing of a man in Woolwich is a "truly appalling murder".
Mr Miliband, cutting short a Germany visit to return to the UK, said his party offered the Government " complete support".
He said: "This is a truly appalling murder which will shock the entire country.
“All of my thoughts are with the family and friends of the victim.
“The British people will be horrified by what has happened in Woolwich. Theywill be united in believing that this terror on our streets cannot be allowedto stand.
“The Labour Party will offer the Government our complete support in establishing the facts of what happened and ensuring that those responsible face the full force of British justice."
The man who filmed the graphic video of a knife-wielding man in Woolwich has explained that he was assured he was in no danger.
The man, who wished to remain anonymous, said the bloodied man came straight for him when he saw he was filming and told him he was not under threat.
He said he was told: "No, no, no - it's cool, I just want to talk to you."
He was initially a little scared but soon realised he would be ok, he explained.
He said the man could have escaped but wanted to explain why he had carried out his actions.
Far-right organisation English Defence League has called on its supporters to gather in Woolwich to protest in response to today's attack.
Leading members have called for people to "take to the streets" and asked them to join a local division of the organisation.
One message, posted online, asked members to get to London "...now if possible."
Deputy Prime Minister Nick Clegg described the Woolwich attack as "shocking and horrifying".
In a statement, he said: "This afternoon witnessed a shocking and horrifying murder on the streets of our capital.
"The whole nation's thoughts will be with the family and friends of the victim this evening.
"People across our country, whatever background they are from, will be united in condemning this attack and we will stand together, as we always do, in our response."
The man who filmed the video of a knife-wielding man in south London said he couldn't believe that the man and acquaintance had not attempted to flee.
"I think he was there just for the police and the army guy," said the witness, who asked to remain anonymous.
"The time it was taking the police to come, that was 30 minutes, and in 30 minutes the guys could have run, take the train, gone anywhere," he added.
Home Secretary Theresa May has confirmed that security around Army barracks across London has been increased.
– Home Secretary Theresa May
The police and Security Service are establishing the full facts of this barbaric case, but there is a strong indication that it was an act of terrorism.
In the meantime, security has been increased at Army barracks across London.
This attack was an attack on everyone in the United Kingdom, and it will be condemned by people from every community.
Our thoughts and prayers tonight are with the victim and his family.
We have seen terrorism on the streets of Britain before, and have always stood firm against it. Despicable acts like these will not go unpunished.