A massive intelligence investigation is under way to find those responsible for the bomb attacks in London which killed at least 37 and left 700 injured.
Many were left in a state of shock
Home Secretary Charles Clarke said looking for potential bombers was like searching for "needles in haystacks".
The Queen and Prince Charles are visiting casualties, while Tony Blair prepares for the G8 summit's last day.
Transport operators ran a near-normal service on the Tube on most lines, but there were fewer commuters than usual.
Of the 300 people admitted to hospital on Thursday, about 80 remain, either convalescing after emergency surgery or too badly burned to return home.
Mr Clarke said 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 being taken seriously.
Scotland Yard have confirmed seven people died in the Liverpool Street explosion and another seven were killed at Edgware Road. At least 21 lost their lives at the King's Cross blast.
Police have been unable to establish whether a suicide bomber was responsible for the attack on a bus in Woburn Place that claimed at least two lives.
But newspapers quoted an eyewitness who got off the bus moments before the blast who saw an agitated man in his mid-20s fiddling with his bag.
Richard Jones, 61, an IT consultant, told the Daily Mail: "This chap started dipping down into his bag and getting back up. He did it about a dozen times in two or three minutes and looked extremely agitated."
The nature of the other three bombings is still unclear.
Mr Clarke has said the death toll is expected to rise. Police are likely to release more details about casualties and the hunt for the bombers later.
They have indicated there will be a more visible police presence in the capital on Friday.
The home secretary said the failure to predict Thursday's bombs should not obscure past successes.
But he admitted: "It certainly was a failure of intelligence in the sense that we didn't know this was coming.
"But by definition when you're looking for needles in haystacks you can miss the needles and the tragedy of yesterday is that we did miss the needles."
Police have said they had no warning of any attacks, or intelligence about any other imminent threats.
And the home secretary defended the recent downgrading of the terrorism threat from severe general to substantial.
Foreign Secretary Jack Straw said Thursday's attacks bore all the hallmarks of the al-Qaeda network.
Anyone worried about relatives or friends they have not heard from is advised to contact a special police hotline on 0870 156 6344.
London was starting to return to normal on Friday, with much of the transport system running as normal.
But the Circle and Hammersmith and City lines will remain closed. Parts of the District line, Metropolitan line and Piccadilly line are suspended.
King's Cross station was due to re-open for suburban rail services only.
Buses are expected to operate normally, except around the immediate areas where the explosions took place.
Transport officials have urged passengers to remain vigilant and keep hold of their baggage at all times.
The prime minister, who left the G8 summit to meet police and security officials in London after the attacks, is due to get back to business with other world leaders later.
Between Aldgate and Liverpool Street tube stations
Between King's Cross and Russell Square tube stations
At Edgware Road tube station
On bus at Tavistock Square