Large numbers of casualties have been reported after at least six explosions on the Underground network and a double-decker bus in London.
Scotland Yard confirmed one of several reports of explosions on buses in the city - in Tavistock Place - but said the cause was not yet known. UK Home Secretary Charles Clarke said several explosions in central London had caused "terrible injuries".
All Tube services and bus services in central London have been suspended.
"The health services are in support to deal with the terrible injuries that there have been," Clarke told reporters outside Downing Street.
Metropolitan Police Commissioner Sir Ian Blair said there had been at least six explosions, but said the picture was still "very confused".
Scotland Yard said explosions have been reported at Edgware Road, King's Cross, Liverpool Street, Russell Square, Aldgate East and Moorgate.
Number 10 said it was "still unsure" whether the explosions were a terrorist attack and although casualties were reported, no further details were yet available.
Prime Minister Tony Blair is to make a live televised statement on the explosions in London at 1200 BST, Downing Street added.
All London Underground services have been suspended indefinitely and bus services in central London (Zone One) have been halted.
A spokesman for Vodafone said emergency services were being given priority access to the mobile phone network which was causing problems for other users.
One caller to BBC Five said his friend had seen "the bus ripped open like a can of sardines and bodies everywhere".