A massive hunt is under way for four would-be suicide bombers who tried to set off four explosive devices on three London Tube trains and a bus.
Police will focus on forensic evidence from the scenes
Streets were cordoned off, parts of the transport network closed and stations evacuated, but no-one was badly hurt.
Three of the devices are the same size and weight as those used for the 7 July London bombings. The same chemicals also appear to have been used.
Analysts say evidence from the scenes may hold clues to the previous attacks.
Thursday's attacks began at about 1230 BST, with bombs at Warren Street station, central London, Shepherd's Bush station in the west, Oval in the south and on a bus in Shoreditch, east London.
It is thought the detonators went off, causing small blasts, but failed to detonate the bombs themselves. The bombers managed to flee.
CCTV pictures from the scenes are expected to be released soon.
Police will focus on forensic evidence from the scenes as well as CCTV footage.
As on 7 July, the bombs are believed to have been carried in rucksacks. Three are thought to have been of a similar size to the previous bombs, while the fourth was smaller and appeared to have been contained in a plastic box.
BBC security correspondent Gordon Corera said it was too early to determine how closely connected Thursday's bombings were with those of 7 July.
However, he added: "Whilst the bombs of two weeks earlier had to be reconstituted from fragments, the new explosive devices will yield clues to the origin and manufacture much faster.
"This will also make it possible to see how similar the explosive is to that found in previous blasts or that found in Leeds or Luton."
Meanwhile, commuters faced difficult journeys to work with some roads still closed early on Friday.
Transport for London service director Nigel Holness told BBC Radio 4's Today programme good services were running on all Tube lines except where they remained suspended after the 7 July bombings.
"We have the maximum deployment possible of the British Transport Police," he added.
"We have started to use sniffer dogs around the Underground.
"And we are looking at deploying staff in different ways."
Mr Holness told Today it would be impossible to "search and scan" all Tube passengers.
"We have three million journeys on the Tube every day and half a million people using the Tube during peak periods."
"We have to keep moving."
Two men, one arrested in Whitehall, close to Downing Street, and another around Tottenham Court Road, have been released without charge.
A wealth of witnesses have also come forward with first hand accounts of events, descriptions of the bombers and photographs of scenes.
Some said the attackers seemed "scared" or "surprised" when their bombs failed to explode properly.
Oval attack witness Kate Reid describe hearing a "pop" as if a big balloon had burst before seeing a young, dark-skinned man with a bag at his feet looking "really scared".
People were kept well away from the sites of the attacks
Met Police Commissioner Sir Ian Blair said the fact devices remained unexploded at the scenes would be "very helpful".
"Clearly the intention must have been to kill. You don't do this with any other intention," he told a press conference on Thursday.
Sir Ian said there was a "resonance" with the 7 July attacks but it was too early to tell if they were linked.
Former government intelligence analyst Crispin Black said the chance to examine the bombs themselves was "forensic bingo", saying: "This is as good as it gets."
As investigators swung into action, Prime Minister Tony Blair urged Londoners to go about their business as normal, saying: "We have just got to react calmly."
World leaders joined in condemning the attacks and urging unity in the face of terrorism.
Details started emerging on Thursday afternoon about the events at each of the attack scenes:
- Warren Street: Witnesses hear a bang at the front of a train and some passengers panic. Armed police are deployed at nearby University College Hospital after a man was seen running there from the station.
Two people are arrested nearby but are later released. A third man arrested in Tottenham Court Road is also released without charge.
- Oval: 20 or 30 passengers evacuated from a train after reports of a bang and some smoke. There is one report of a suspect trying to use a handgun to detonate explosives in a rucksack. Several witnesses report bystanders trying to tackle a man as he flees the station.
- Shepherd's Bush: A man reportedly flees after an attack on a Hammersmith and City Line train.
- Shoreditch: An explosion blows out the windows of a number 26 bus on Hackney Road but causes no structural damage.
Police have asked that any images of the attacks are sent to www.police.uk. The hotline number for anybody with information is 0800 789 321. Witness reception points have been set up near the four scenes.