Because of this notorious past, law enforcement officials in the British capital have taken many steps to ensure the safety of its citizens. There is now a "Ring of Steel" around the center of London, a series of safety measures designed to deter – and prevent – terrorism.
They've caught the eye of New York City Mayor Michael Bloomberg, who visited London today to see what he could learn from this "Ring of Steel" system.
The center of London is watched by one million security cameras, covering virtually every square block.
"This is what it's all about, being able to deter, prevent and, if God forbid, something happens, apprehend whoever caused it," said Bloomberg today.
ABC News was given a rare look inside the system today, visiting a control room where police can access 13,000 cameras covering London's subway system at the touch of a button.
"No place to hide," a technician told us.
British Transport Police control rooms are in every city and every major town across the U.K., monitoring streets, train stations and public landmarks. The goal: to gather intelligence and monitor potential terrorist suspects.
Other cameras capture license plates and drivers' faces. Overall, the average London resident is seen on video an estimated 300 times a day.
Forty Cameras on a Half-Hour Trip
We tested it ourselves. On a short trip to the ABC News bureau this morning, our crew was seen by cameras on a street, while passing a restaurant, in a store, on the subway, on a crosswalk, and finally while entering our office building. In total, 40 cameras spotted us in less than half an hour.
The nearly seamless network enables police to solve terror attacks with remarkable speed. The bombers who killed 52 people on the London subways in 2005 were filmed every step of the way. Copycat bombers were caught after cameras captured images of them fleeing their targets.
Critics argue cameras are better at recording attacks than preventing them, but Mayor Bloomberg said today they might make terrorists think twice.
"Cameras help after the fact and they help as a deterrent," he said.
As intrusive as the cameras are, polls show large majorities in Britain support them. They say they see the forfeiture of some privacy as an acceptable price for safety.