Police leaders say they will not abandon their "shoot-to-kill" policy and warn more innocent people could be killed in the fight against terrorism.
The message came after Brazilian Jean Charles de Menezes, 27, was shot dead by officers in London after being mistaken for a suicide bomber.
Met Police Chief Sir Ian Blair said "shoot-to-kill in order to protect" would continue, despite the "tragedy".
Police are still questioning three men in connection with Thursday's attacks.
Sir Ian has apologised for the killing of Mr Menezes, but defended the actions of his officers.
The shooting is being investigated by Scotland Yard's Directorate of Professional Standards and the Independent Police Complaints Commission.
Celso Amorim, Brazil's foreign minister, said Jack Straw had promised a full investigation into Mr Menezes' death.
Mr Amorim, who will meet Mr Straw on Monday, said: "We can't recover the life of the Brazilian citizen who has been killed, but we can discover the details.
"The Brazilian government and the public are shocked and perplexed that a peaceful and innocent person should have been killed."
Brazilian authorities say they want to know why the 27-year-old was considered to be a suicide bomber.
JEAN CHARLES DE MENEZES
Born 07/01/78, a Brazilian national
Originally from the town of Gonzaga, 500 miles northeast of Sao Paulo in the south-eastern state of Minas Gerais in Brazil
Moved to Sao Paulo at age 14
Lived in London for three years, working as an electrician
John Denham, chairman of the Home Affairs Select Committee, said the investigation must be carried out quickly and the conclusions made "very public".
In an interview on BBC Radio Four's Westminster Hour Mr Denham, a former Labour minister for policing, said a previous review into fatal shootings blamed poor intelligence or briefing of frontline officers for the deaths of innocent people.
"What we need to know really is whether in this case there's something fundamentally different to other shootings, or whether it's going to be in practice, those same failings."
Mr Menezes, from Tulse Hill, south London, was killed at Stockwell Tube station on Friday. He was not connected to Thursday's attempted bombings.
Meanwhile, a third man was arrested by police under the Terrorism Act. He was arrested in Tulse Hill on Saturday evening.
Police have been granted more time to question two other men arrested in Stockwell under the Terrorism Act.
Officers are still trying to trace the four men suspected of trying to bomb the Tube and bus network on 21 July.
Sir Ian said there was no reason to believe the suspects have left the country.
Detectives believe it is possible that one or more of the men may have killed themselves.
Sunday's other developments include:
Alex Pereira retraces the final movements of Mr Menezes, his cousin, in an emotional protest.
Police believe two of the 7 July suicide bombers may have met terrorists involved in the failed 21 July attacks at a Welsh whitewater rafting centre.
Met Police deputy assistant commissioner Brian Paddick met community leaders in Stockwell to discuss the shooting.
Searches continue in Little Wormwood Scrubs where a package, possibly linked to the failed attacks, was found. It was removed for forensic examination after several controlled explosions.
Officers still searching an address raided on Saturday in Streatham Hill, south London, in connection with the attempted bombings.
Brazil's foreign minister Celso Amorim met Foreign Office officials in London seeking an explanation for the shooting. He will meet Foreign Secretary Jack Straw on Monday.
Home Secretary Charles Clarke delays his holiday.
Aldgate Underground station - where seven commuters and suicide bomber Shehzad Tanweer were killed on 7 July - will reopen on Monday.
Sir Ian, defending the actions of his officers, said: "What we have got to recognise is that people are taking incredibly difficult, fast-time decisions in life-threatening situations.
"It wasn't just a random event and what's most important to recognise is that it's still happening out there.
"Somebody else could be shot but everything is done to make it right.
"The important thing is there's nothing gratuitous going on, there is nothing cavalier here, there is no conspiracy to shoot people."
Sir Ian said officers had to aim for a suspected suicide bomber's head as a body shot could trigger an explosion as the chest area is where explosives are most likely to be.
Home Secretary Charles Clarke described the shooting of Mr Menezes as an "absolute tragedy".
But he said: "I hope [the family] understand the police were trying to do their very best under very difficult circumstances."
On the ongoing bombings investigation, Mr Clarke said "good progress" was being made thanks to "tremendous support" from the public.
Mr Menezes's family is struggling to come to terms with his death.
Mr Pereira, from London, told the BBC: "Apologies are not enough. I believe my cousin's death was result of police incompetence."
Describing his cousin as a "person full of life" he said he had been "a victim of government's mistakes".
His grandmother, Zilda Ambrosia de Figueiredo, told Globo TV "there was no reason to think he was a terrorist".
The body of Mr Menezes is to be taken back to Brazil as soon as possible.
1: Jean Charles de Menezes leaves a house under surveillance and arrives at Stockwell station
2: Witnesses say he vaults the automatic ticket barriers and heads for the platforms
3: He then ran down an escalator after being approached by up to 20 plain-clothed police officers and tried to board a train
4: He apparently refuses to obey police instructions and after running onto a northbound Northern line train, he is shot dead