Four policemen who were stabbed as they tried to arrest a man who had grabbed a 12in knife from a butcher's shop have been praised for their "outstanding bravery".
The officers were injured as they attempted to wrestle the weapon from the man's grasp.
Eyewitnesses told how police chased the man down the road and in and out of stores before he grabbed the knife.
After a violent struggle he was arrested on suspicion of attempted murder.
The drama brought chaos to Kingsbury, in the suburbs of north west London yesterday morning.
Commander Christine Jones, of the Metropolitan Police's territorial policing unit, said: "These officers put their lives on the line by confronting this man in what was an extremely violent and spontaneous situation.
"The four officers have demonstrated outstanding bravery and this once again highlights the dangerous situations our officers deal with on a daily basis as they carry out their duties keeping Londoners safe."
All four policemen were taken to hospital but one, in his 20s, was later released after being treated for a broken hand.
Of the three men who remain in hospital, one, in his 20s, suffered a stab wound to his stomach; a second, also in his 20s, suffered head injuries and a stab wound to the arm; and the third, in his 30s, is being treated for a stab wound to the leg.
Home Secretary Theresa May said that she and Minister for Policing and Criminal Justice Nick Herbert had passed on their sympathies to the wounded officers through the Commissioner of the Metropolitan Police, Bernard Hogan-Howe.
She said: "The bravery demonstrated by these officers is a vital reminder of the debt we owe the police, who put themselves in harm's way everyday to protect the public.
"Nick Herbert and I have spoken to the Commissioner to pass on our deep sympathies for the severe injuries suffered and our immense gratitude to the officers for their actions. We wish them a full and fast recovery."
Scotland Yard said officers were called at around 8.40am yesterday to reports of a man causing a disturbance in the street.
"Officers attempted to speak with the man before he ran into a local butcher's shop and grabbed a knife," a spokesman said.
"Officers followed the man in an attempt to detain him and were subsequently assaulted.
"Four male police constables suffered stab injuries during the incident and have been taken by the London Ambulance Service to hospital.
"A 32-year-old man has been arrested on suspicion of attempted murder and remains in custody at a north London police station."
The incident ended at the Kingsbury Halal Butchers, 100 yards from Kingsbury Tube station.
Mohammad Qasim, 51, owner of the store, described how a man walked in and said: "Give me a chopper."
Mr Qasim, who has owned the shop for 24 years, said: "It was about 9.20, everybody was busy, customers come in the morning time.
"The man came quickly inside and said 'Give me a chopper'.
"The boys who work for me said 'Why give you a chopper?'. The man was very strong."
The shopkeeper said the man then took a butcher's knife which was about 12in (30cm) long.
He said: "After a few minutes police came in and tried to catch him and he stabbed one.
"There was plenty of blood inside, everything was a mess and then police came and said 'Go out'.
"Everybody came out and police went in and took the guy."
Asked how he was feeling, Mr Qasim said: "Very bad, I'm very upset."
Another local shopkeeper, Girish Modha, said: "A man was shouting at police in a small alleyway next to a hairdresser's shop which neighbours mine.
"He grabbed a piece of fluorescent tubing and brandished it at police. He then ran down Kingsbury Road, going into a cash-and-carry shop. At one point I think he threw a brick and smashed a police car window.
"He then ran round the roundabout and carried on towards the Tube station. He went into a butcher's, got a knife and that's when the stabbing took place."
London Mayor Boris Johnson said: "Our immediate thoughts are with the injured officers and their families and colleagues.
"This was an appalling attack on brave individuals who work tirelessly to protect our communities and keep our streets safe.
"I have asked to be kept updated about how the four officers are progressing and wish them all a full and speedy recovery."