The co-ordinated assault mirrored the 7 July bombings, which left 56 dead including the four suicide bombers. This time, the devices, three on Underground trains and one on a bus, failed properly to ignite.
At least three of the bombers were believed to be still at large last night, despite passengers wrestling with one of them. It was unclear whether one man being treated at University College Hospital, where there was a massive police operation yesterday, was linked to the attacks.
The devices were on Tube trains at the Oval, Warren Street and Shepherd's Bush, and on a No 26 bus in Shoreditch.
As on 7 July, the targets were in the north, west, east and south of the city.
Police and intelligence services were investigating whether this may have been a less effective copycat exercise by a more amateur group of Muslim hardliners than the 7 July bombers. But there were indications that the devices, though smaller, were made up of similar explosive to the bombs two weeks previously, suggesting that it was an attack by a cell linked to the 7 July bombers that went wrong.
That eventuality would shatter the hopes of millions of Londoners that the attacks two weeks ago were a one-off.
Despite the intensive efforts of police and the security services, yesterday's attacks were evidence of a further lack of intelligence about potential bombers.
Sir Ian Blair, the Metropolitan Police Commissioner, said that forensic investigations indicated that there had been four bombs which had failed to explode when detonated. He said: "Clearly, the intention must have been to kill. You do not do this with another intention."
The discovery of backpacks containing explosive material will provide police with a large amount of forensic information, allowing comparison with fragments recovered two weeks ago. Initial investigations were said to show marked similarities, raising fears that a bomb-maker, possibly al-Qa'ida trained, remains at large.
Although there were reports that two people had been arrested one man was led away by armed police in Whitehall, and another detained near Warren Street police said that they were not related to the incidents.
Sir Ian said there was a "resonance" between the attacks, and the similarities between 7 July and yesterday were almost immediately apparent.
The Tube bombs were on the Victoria line at Warren Street, on the Northern line at Oval, and on the Hammersmith and City line at Shepherd's Bush, while the bus bomb was in Shoreditch, in east London north, south, west and east, the same points of the compass referred to in the statement claiming responsibility for the 7 July bombings.
Three of them were on Underground trains and the fourth on the top of a No 26 bus, which, like the bomb on the No 30 bus in Tavistock Square, detonated about an hour later. Both had the destination of Hackney on the front.
The incidents ratcheted up the fear factor in a city only just getting its nerve back. Some eyewitnesses to the Tube evacuations spoke of "panic" among passengers fearing carnage.
In their public statements yesterday, both Tony Blair and Sir Ian were anxious to reassure the public; similarly, Underground staff had clearly been instructed in the importance of remaining calm to counter the panic effect of further incidents.
The first reports of explosions came just after 12.30pm on a day when, ironically, the London Evening Standard newspaper was reporting that the city was "learning to smile again".
At Shepherd's Bush station in west London, just before 12.30pm, the area was evacuated after a suspicious rucksack was discovered on the station. The entire area was cleared and searches were still continuing there and at all the other scenes last night. It was not clear whether there was an explosion.
However, at both Oval and Warren Street there were reports of small explosions over the next 15 minutes.
One woman, Andrea, who was at Oval station in south London, said: "I was in a carriage and there was a big bang, it sounded like a balloon had popped but a lot louder, and then we all moved to one end of the carriage. There was something on the floor, you could see something had exploded. They opened the door so we could move through into the next carriage, and there was a guy just standing in the carriage. We pulled into Oval and we all got off on to the platform. The guy just ran and started running up the escalator. Everyone was screaming to stop him."
Ivan McCracken, a passenger on the Victoria line at Warren Street, told Sky News: "I was in a middle carriage and the train was not far short of Warren Street station when suddenly the door between my carriage and the next one burst open and dozens of people started rushing through. Some were falling, there was mass panic.
"When I got to ground level there was an Italian young man comforting an Italian girl who told me he had seen what had happened.
"He said that a man was carrying a rucksack and the rucksack suddenly exploded. It was a minor explosion, but enough to blow open the rucksack. The man then made an exclamation, as if something had gone wrong. At that point, everyone rushed from the carriage."
As emergency services went on to full alert and police cordons were erected around the areas, events took on an alarmingly familiar pattern to those just two weeks ago. Most of the Underground system ground to a halt as a Code Amber alert, ordering drivers to pull into stations and unload trains, was issued.
Almost exactly an hour later, at 1.30pm, the driver of the No 26 bus, travelling along Hackney Road in Shoreditch, heard a bang on the top deck. He stopped the bus and discovered that the windows on the top deck had been blown out; there was also a smell of burning, according to eyewitnesses. He evacuated the passengers and called the emergency services. Police threw a cordon around the area.
By now, Tony Blair, who had earlier met officials to discuss anti-terrorism measures, and was having lunch with the Australian Prime Minister, John Howard, was notified; the Cabinet's Cobra committee was convened. He cancelled all engagements, prepared for the worst.
However, it soon became clear that the physical effects of the bombs were minimal. By mid-afternoon, the earlier warnings by police to the public to "stay put" and wait for developments were replaced by an assurance from Sir Ian that the "situation is under control" and that the capital's workforce could head off home by whatever transport was available.
The psychological effects on London may be more acute and long-term. While the public face was one of calm and reassurance, privately, both the Government and senior police officers were said to be desperately anxious about the effect on public confidence in their own safety in the nation's capital. Though unconnected to the attacks, the arrest at gun-point of a man at the gates of Downing Street illustrated the tension. Last night, there was another large evacuation in Portland Square, north of Oxford Street.