UK Prime Minister Tony Blair said it was "reasonably clear" there had been a series of terrorist attacks.
He said it was "particularly barbaric" that it was timed to coincide with the G8 summit. He is returning to London.
An Islamist website has posted a statement - purportedly from al-Qaeda - claiming it was behind the attacks.
London's police chief Sir Ian Blair said there had been "many casualties" but it was too early to put a figure to those killed or injured.
More than 100 casualties and one of the dead were taken to the Royal London Hospital in Whitechapel.
And St Mary's Hospital said it was dealing with 26 injured people, including four with critical injuries and eight in a serious condition.
It's particularly barbaric that this has happened on a day when people are meeting to try to help the problems of poverty and Africa
Tony Blair
Blair statement in full
Sir Ian urged people to stay where they were and not to call emergency services unless it was a life-threatening situation.
He reassured the public that an emergency plan was in place and the situation was "steadily coming under control".
Scotland Yard said explosions have been reported at Edgware Road, King's Cross, Liverpool Street, Russell Square, Aldgate East and Moorgate.
All London Underground services have been suspended indefinitely and bus services in central London (Zone One) have been halted.
Early reports had suggested a power surge could be to blame for explosions on the Underground but this was later discounted.
Describing the bus blast in Tavistock Square, eyewitness Belinda Seabrook said she saw an explosion rip through the bus.
Source: BBC.com.
Can't say I didn't see it coming. I hope every Brittish from this board is alright though.