Sir Ian Blair acknowledged that the death of the Brazilian electrician Jean Charles de Menezes, killed last week by undercover police, had been a "dreadful mistake".
But he said it should not divert his officers from the "main issue" of combating terrorism.
Asked whether he could guarantee that a similar tragic accident would not happen again, he told Channel 4 News yesterday: "I can't in any absolute sense, but I know there have been 250 incidents since the 7 July where we considered whether we were being confronted by suicide bombers."
Indicating a small gap between his fingers, he said: "There have been seven times when we have got as close to calling it as 'that' and we haven't. An officer is faced with a decision as to whether to call a particular operation. That has happened seven times."
A union official said yesterday that police officers had put guns to the head of the driver of the Tube involved in the Stockwell shooting of Jean Charles de Menezes. According to the official, the driver of the train tried to run into a tunnel after hearing the shots and seeing passengers flee the train, but officers dragged him back to the platform, threw him to the floor and threatened him with their pistols.
The driver has been off work since last Friday. The train drivers' union Aslef is considering a submission to the Independent Police Complaints Commission.
Underground staff on the Northern line thought that police had been "gung-ho" during the shooting, said Steve Grant, a senior official at Aslef.
They believed that the driver's colour - he is of west African descent - may have influenced the officers' behaviour. Two of the suspected bombers who tried to target London the previous day are African.
When the driver got back to the depot and told his colleagues what happened, some of them refused to take trains out.
Another Northern line driver held at Stockwell station at a red signal on the other south-bound track at the time of the incident had no idea what was going on and could not contact controllers either on his cab radio or via a phone on the platform, Mr Grant said. Even a "May Day" alarm failed to elicit a response, according to Mr Grant.
The Brazilian lawyer who will represent the family of Mr de Menezes said he would be seek around $500,000 (£287,000) in compensation from the British government. "I was with the family yesterday and the mother is completely shell-shocked as you might imagine," he told The Independent.
The lawyer now plans to mount a civil action on the behalf of the de Menezes family. He said the action would be for "homicídio" (murder).
"An innocent man was gunned down without reason by the British police and there must be some form of adequate compensation," he said.
The killing sparked angry protests in Mr de Menezes' hometown of Gonzaga, where some 700 demonstrators took to the streets. And a group of 16 protesters, linked to the Landless Rural Workers Movement (MST) hung a sign on the fence around the British embassy in Brasilia yesterday blaming "state terrorism" for the killing of Mr de Menezes.
"We're outraged. We have to tell Brazil and the world that it's unacceptable to shoot first and ask questions later," said Altacir Bunde, a spokesman for MST.
The death of Mr de Menezes is being investigated by the Indepedent Police Complaints Commission, but its inquiry could take several months.
The electrician was tailed from his home in Tulse Hill, south London, to Stockwell Tube station by undercover officers. When they told him to stop, he vaulted the ticket barriers and ran on to a train. He was killed after being shot eight times.Reuse content