More than four thousand people have been stopped and searched in London in the past two weeks as police try to address public concern over young people carrying knives.

The £1 million operation has led to the seizure of almost 200 weapons and to 210 arrests. Officers have used airport-style search arches and metal-detecting wands on the streets.

The Home Office announced an advertising campaign yesterday with shocking images of the damage that can be done with a knife. One shows a man with a knife sticking out of his stomach, and another shows a hand from which the thumb has been severed.

Although the campaign highlights injuries caused by a Swiss Army knife, police have revealed that the majority of weapons seized from teenagers had been taken from their kitchens. The law forbids anyone under the age of 18 from buying a knife of any kind.

In Operation Blunt 2, senior officers have taken the unusual step of authorising emergency powers, known as Section 60 orders, almost 100 times to give themselves the power to stop anyone even without reasonable suspicion. Some of their tactics have been questioned by members of the Metropolitan Police Authority.

Members of the watchdog warned Sir Ian Blair, the Metropolitan Police Commissioner, yesterday that rude and aggressive police officers were endangering community support. They were also concerned at the wide use of Section 60 powers.

But Sir Ian vowed that the emergency powers would continue to be used until the violence eased.

John Roberts, an independent member of the body, said that the searches were supported but only when done with sensitivity and respect. He said that not all frontline officers were doing this and they should be sent for training or sacked.

He added: “We must get these officers retrained or out of the Police Service because they are the ones doing damage to your credibility and that of the police.”

Jenny Jones, a Green London Assembly member, said that she wanted to know how Sir Ian could justify a long-term Section 60 operation when crime figures were falling. She said: “If we are operating aggressive policing it is going to take considerable resources to allay community fears.”

Section 60 powers have to be authorised by a senior officer if they believe that “incidents involving serious violence may take place in his/her police area and that an authorisation will be effective in preventing these incidents”.

Speaking at the meeting of the authority, Mr Blair said: “We do not accept that this is an aggressive tactic. It is robust because it has to be. The message to officers is clear, ‘This has got to be done with respect’. But search arches do give us a very significant ability people through very fast. It is equivalent to an airport search. We put these in places where people have not got much time to think that they are not going through them.

“If they turn away that is of great interest to us and we go and have a conversation with them.”

He added: “[Operation] Blunt will stop when the murders start to reduce because we just cannot go on at this level of deaths in this age group in a city where crime in terms of violence is falling. This outweighs all the other successes of the Met.”

Operation Blunt 2 is under way in all 32 London boroughs, but extra resources are being sent to the ten worst-hit areas including Lambeth, Southwark and Croydon.

Since the operation started two weeks ago officers have used airport-style search arches 54 times, implemented Section 60 powers 95 times and carried out 51 weapon sweeps.

In total there have been 4,277 stop-and-searches and 210 people have

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