Police, fire and ambulance services, health chiefs and transport operators immediately activated a long-rehearsed, but previously theoretical, procedure for reacting to a strike.
The emergency team, led from Scotland Yard by the Metropolitan Police's Westminster commander, made swift decisions. The Tube network was shut down and all bus services in central London halted shortly afterwards. The capital's hospitals were put on disaster alert and told to cancel routine operations.
Several mainline stations were closed and an air exclusion zone imposed over much of the city. Security was stepped up at obvious terrorist targets, such as the Houses of Parliament and Buckingham Palace.
A quarter of a mile away in Whitehall, the Home Secretary, Charles Clarke, summoned a meeting of the Cobra, the Government's crisis management team. It is named after Cabinet Office Briefing Room A, where the committee meets.
Joining Mr Clarke around the table were cabinet colleagues, Eliza Manningham-Buller, the director-general of MI5, Bill Jeffrey, the security and intelligence co-ordinator in the Cabinet Office, as well as other senior police and security officers.
Its immediate task was to decide whether to invoke emergency powers, such as ordering evacuations or calling in the military. It decided against such dramatic steps, judging for the moment that the operation led by Scotland Yard was adequate for dealing with the crisis.
The immediate challenge facing the emergency services, who called in reinforcements from across the south-east, was to remove the seriously hurt from the four bomb blast sites. Those with the worst injuries were carried by helicopter to hospital, with others transferred in the 100 ambulances that were made available. Double-decker buses were requisitioned to carry the "walking wounded" for treatment. Once the injured were removed or treated on site by paramedics, detectives moved in to search for evidence.
Emergency planners were well aware that sensitive media management was also crucial to handling the crisis.
As news of the blasts spread across the country, and hundreds of thousands of people tuned in to television and radio news programmes, police chiefs were at pains to calm public fears, repeatedly stressing the situation was being brought under control. Sir Ian Blair, the Metropolitan Police commissioner, toured the TV studios to provide reassurance to the public as he emphasised a "sophisticated" emergency plan was being implemented.
A news centre was set up and a news conference arranged to get messages across to Londoners. They included pleas to stay out of London if possible, to stagger journey times home and to only dial 999 in a life-threatening situation.
Tony Blair flew from the G8 summit at Gleneagles to chair a second Cobra meeting yesterday afternoon, at which the handling of the disaster was examined. The likely identity of the attackers was also discussed, with those present concluding that the co-ordinated blasts bore all the hallmarks of al-Qa'ida.
The emergency plans for London have been overhauled since the attacks on New York and Washington nearly four years ago. The capital had lived with two decades of IRA attacks, culminating in the Canary Wharf blast in February 1996. It has also coped with a series of disasters, the most recent being the Paddington rail crash of 1999.
But never before had London faced the prospect of a co-ordinated attack, possibly using deadly chemicals.
As a result the "London resilience partnership", bringing together the Government, the Greater London Authority, local councils, the emergency services and health chiefs, has drawn up fresh emergency plans.
It has compiled a range of scenarios, such as coping with the aftermath of a chemical, biological or nuclear attack. Plans are in place to evacuate hundreds of thousands of people out of the capital and move the seat of government out of Whitehall.
Emergency services have conducted regular exercises in London, including a simulated "chemical strike" on an Underground train at Bank station in September 2003. It concluded evacuation plans for stranded passengers were inadequate and warned of communication problems underground.
The early evidence yesterday was the emergency response had gone as well as could be expected. Mike Grannatt, a former head of the Cabinet Office Civil Contingencies Secretariat, told the BBC: "Since 9/11, the planning for actually handling mass casaulties, for handling widespread events has improved.
"You have seen some of that, the reports for example of buses being used to ferry casualties, the way in which the Underground and the buses have stopped quickly so things can be checked, that kind of response has been well-polished."