There have been a series of explosions on the London Underground and up to three on buses in the city.
The entire tube network has been closed and union officials have said their sources have told them there was at least one explosive device on the underground.
An explosion was reported at 8.49am at Liverpool Street station and emergency services were immediately sent to the scene.
Explosions have also been reported at King's Cross and Aldgate and an eyewitness said she saw an explosion rip though a bus near Russell Square.
Belinda Seabrook said: "I was on the bus in front and heard an incredible bang. I turned round and half the double decker bus was in the air."
London Underground said: "The network has been suspended until further notice and all stations are being evacuated.
"This follows major incidents at Liverpool Street and Edgware Road stations. Emergency services are on site and are working closely with London Underground."
A London Ambulance Service spokeswoman said: "We are sending multiple resources to a number of central London locations."
A spokesman for the City of London Police said there were "quite a large number of casualties" at Aldgate Station.
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