Four police officers have been stabbed after being called to a disturbance in north-west London.
Witnesses said a man had been chased by police in and out of shops before he fled into a butchers, grabbed a knife and attacked the officers.
Two of the PCs were seriously hurt and one suffered a broken hand during Saturday's incident in Kingsbury Road.
A 32-year-old man was arrested on suspicion of attempted murder.
Ch Supt Dalwardin Babu, the borough commander for Harrow, said one of the officers was stabbed in the stomach, a second suffered head injuries and a stab wound to the arm, the third officer was stabbed in the leg and the fourth officer, who was slashed, suffered a broken hand.
'Give me a chopper'
All four are based in Harrow. The PC who was stabbed in the stomach is in his 20s and has been in the force for three years. The officer, who suffered head injuries and was stabbed in the arm, has been with the service for six years and was also in his 20s.
The officer who was stabbed in the leg has been in the police for five years and is aged in his 30s.
The PC, who suffered a broken hand, is in his 20s and has been in service for eight years. He has been released from hospital but the other three remain in hospital.
London Mayor Boris Johnson said: "This was an appalling attack on brave individuals who work tirelessly to protect our communities and keep our streets safe."
Metropolitan Police Commissioner Bernard Hogan-Howe said: "It was shocking that on a hopefully quiet Saturday morning in a London residential area we had such a terrible incident.
"It just shows the difficult job that police officers do."
Mohammad Qasim, 51, owner of the Kingsbury Halal Butchers for 24 years, described how the attacker walked into the shop and demanded they hand over a knife.
Mr Qasim said: "The man came quickly inside and said: 'Give me a chopper'.
"The boys who work for me said: 'Why give you a chopper?'"
He said the man then grabbed a 12in long knife.
"After a few minutes police came in and tried to catch him and he stabbed one.
"Everything was a mess and then police came and said 'go out'.
"Everybody came out and police went in and took the guy."
Shopkeeper Girish Modha, who owns a nearby sweet shop, described the events leading up to the stabbings.
"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 butchers, got a knife and that's when the stabbing took place."
The normally busy shopping street by Kingsbury Tube station remained cordoned off on Saturday afternoon.
Ketan Vyas, the manager of a cash-and-carry store, VB and Sons, said a man in his 30s or 40s had burst into the shop chased by police.
Mr Vyas said: "He picked up some cans of beans and threw them at the police and then carried on running out of the store and down the road.
"There were a lot of police after him. He was only in here for a few seconds."
Describing the aftermath of the incident, a worker at a Carphone Warehouse shop opposite Kingsbury Halal Butchers said: "I saw about eight police officers on top of a man.
"Ambulances arrived to take away the injured policemen and the man was also taken away."
Barry Gardiner, MP for Brent North, who visited the area following the stabbings, said he was "deeply upset" and the mood in the area was "very sombre".
Labour's mayoral candidate Ken Livingstone said: "We all know that knife crimes and stabbings are still a problem but even with that knowledge this attack is shocking."
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."
Kingsbury Circle has been closed in all directions following the incident and five bus routes have been diverted.