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More than 50 people died in the London bomb attacks, the head of the Metropolitan Police has said.
Although only 49 people are confirmed dead, Sir Ian Blair said an unknown number of bodies remain in the blast-hit Tube train at Russell Square.
The police had an "implacable resolve" to track down those responsible for the bombings, the Met Commissioner said.
The police chief confirmed that 13 people were killed in the bus blast at Tavistock Square alone.
There was "absolutely nothing" to suggest that this was the work of a suicide bomber or "to rule it out", said Sir Ian.
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It was "blindingly obvious" that a terrorist cell was operating in Britain, he said, as massive intelligence operation to find the perpetrators moved into gear.
"This was an attack which was entirely arbitrary, random, irrespective of race, of colour of gender and age," he said.
Victims not only came from Britain but from Sierra Leone, Australia, Portugal, Poland and China, he said.
The timing of the attacks meant that one person could not have carried all four attacks out, he added.
Politicians and police have paid tribute to the resilience of Londoners, with transport returning to a near-normal service on the Tube.
Speaking from the G8 summit in Gleneagles the prime minister said the purpose of terrorism was to put anger and hatred in people's hearts.
Meanwhile, the Queen sent a message of defiance to those behind the attacks, saying: "They will not change our way of life."
London Mayor Ken Livingstone said he had believed for some time that an attack on the city was inevitable and said the emergency operation had "worked like clockwork" because of the planning involved.
He said a book of condolences would be opened at City Hall on Monday and that a London's bombings relief fund would be set up to assist those that have suffered.
Both Mr Livingstone and Sir Ian urged people to come forward with information that may help track down the bombers. People can call the Anti-Terrorist Hotline on 0800 789 321.
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Ports and airports are on a state of high alert and a huge recovery of CCTV material has started in an attempt to identify the bombers.
Anti-terrorist branch head Andy Hayman said devices with less than10 lbs (4.5kg) of high explosive were left on the floor in the Tube trains and may have been left on the seat of the bus.
He added that emergency services were encountering difficulties recovering bodies from the Tube train between King's Cross and Russell Square because the tunnel was unsafe.
Home Secretary Charles Clarke said looking for potential bombers was like searching for "needles in haystacks".
A claim on the website of a previously unknown group, the Secret Organisation Group of al-Qaeda of Jihad Organisation in Europe, saying it was behind the blast, was now being taken seriously, he said.
The failure to predict Thursday's bombs should not obscure past successes, he said, but admitted: "It certainly was a failure of intelligence in the sense that we didn't know this was coming."
Passengers evacuate an underground train at Kings Cross (Photo: Alexander Chadwick)
Prince Charles spent more than an hour, along with the Duchess of Cornwall, visiting casualties and staff at St Mary's Hospital near the Edgware Road tube blast.
Scotland Yard confirmed seven people died in the Liverpool Street explosion, another seven at Edgware Road, a further 13 in the Tavistock Square bus blast and at least 21 at the King's Cross blast. A 49th person died in hospital later.
Some people 700 were hurt, 22 of whom were serious or critical.
Anyone worried about missing relatives or friends can contact a special police hotline on 0870 156 6344.