The almost simultaneous explosions rocked at least three London subway trains and ripped apart a double-decker bus during the morning rush hour, according to the police.

    London's streets echoed with sirens and the entire bus and underground transport network was shut down just a day after an overjoyed city celebrating London's successful bid to hold the 2012 Olympics.

    Police said there had been no warning.

    SKY TV film showed a mangled red bus with its top collapsed. Bloody and bandaged witnesses said panicked crowds were fleeing as the blast rained glass and other debris onto the street.

    "It was a massive explosion and there were papers and half a bus flying through the air," a panicked passenger was quoted by Skynews as saying.

    Home Secretary Charles Clarke, Britain's top law enforcement officer, confirmed four explosions: three in the subway system and one on the bus at Russell Square near the British Museum. He said there were "terrible injuries".