A British solider has been killed as he came out of the Royal Artillery Barracks in Woolwich in what is being treated as a terror attack.
Prime Minister David Cameron says it is "shocking", and has called an emergency high-level meeting of the country's top security grouping, COBRA.
CNN's Nic Robertson says it's clear the soldier - from the Royal Artillery Barracks in Woolwich - was specifically targeted.
"The eyewitness says that he was struck by a vehicle, knocked to the ground, two men jumped out of the vehicle and then attacked him."
One witness says a sword or meat cleavers were used.
Shocking video footage has emerged of what is being described as a savage attack.
It shows a man with his hands covered in blood, carrying a knife and a meat cleaver.
A second piece of footage shows the same man lying in the road outside the Royal Artillery Barracks in Woolwich after being shot by police.
CNN's Atika Shubert says witnesses she has spoken to say the soldier was hacked to death.
"There do seem to be a number of weapons that were used in the attack, these large knives.
"Also, according to another eye witness, that one of them had a gun, and that's something that's actually been confirmed by police."
The pavement outside the barracks where the incident happened is awash with blood.
CNN's Nic Robertson says it was originally described as a road traffic accident - now some sources say it is a politically motivated Islamist terror attack.
"We don't know why it has suddenly reached this very high level of importance, that this meeting would be called at this level.
"Is it just because he was a soldier or is there more to it?"
Metropolitan police commander Simon Letchford says a number of weapons were used and armed officers attended.
"Two men, who we believe from earlier reports to have been carrying weapons, were shot by police.
"They have both been taken to separate London hospitals and are receiving treatment for their injuries."
Police have flooded the area and the surrounding roads are all blocked off.
David Cameron has cut short a trip to Paris to return to London after the attack.
Security Analyst Will Geddes believes there is a significant back story to the savage attack of a British soldier in London.
Mr Geddes says the two perpetrators invited members of the general public to take video footage and photos of their actions.
"We know with terrorists these days they're very media savvy. They want to get as much viral publicity, if you like, as they possibly can."
Mr Geddes says inviting the public to do this doesn't fall in line with standard criminal behaviour.
Photo: Getty Images