Prime Minister Tony Blair urged Londoners to return to "business as usual," saying, "We have to react calmly and continue with our business. We know why these things are done - they are done to scare people and make them feel worried."
The incidents took place at the Warren Street and Oval subway stations, an area above ground at Shepherds Bush and on a number 26 bus, en route from Waterloo to Hackney.
"We know that we have four explosions, four attempts at explosions, and it's still pretty unclear as to what's happened," said Metropolitan Police Commissioner Sir Ian Blair soon after the bombs were detonated.
He said the bombs appeared to be small and the situation was coming under control. The attacks, he said, were "close to simultaneous."
Police are appealing to anyone who may have photos or mobile phone images from any of today's incident scenes, either at the time of the incident or shortly before, to send them online to www.police.uk.
London Mayor Ken Livingstone suggested Londoners "go about their business… use the transport system to get home. I once more want to congratulate transport workers and the emergency services for their impressive efforts today, and also the calm of Londoners."
Three subway lines remain closed: Northern, Victoria and Hammersmith and City. Other lines and stations, underground and main line, remain open and functioning.